feat: Add support for CURL networking. (#487)

BEGIN_COMMIT_OVERRIDE
feat: Add support for CURL networking.
feat: Add proxy support when using CURL networking.
END_COMMIT_OVERRIDE

This PR adds an alternate networking implementation which uses CURL
instead of boost beast. Using CURL enables support for all advanced
networking features including support for http, https, socks4, and socks
5 proxies.

Proxy configuration can be done using standard environment variables, or
using the builder for the SDK.

CURL is used via the CURL multi interface combined with boost::asio to
provide non-blocking IO. https://curl.se/libcurl/c/libcurl-multi.html

This allows for the CURL requests to be driven similarly to the
boost.beast networking which is additionally supported.

CURL support is conditional based on the LD_CURL_NETWORKING CMake
option. When it is enabled all HTTP/HTTPS request in the SDK use CURL.
This support does not affect the redis persistence implementation.

The CI and release process has been updated to build and test the CURL
and boost.Beast versions of all of the libraries. These changes
constitute a non-trivial portion of the PR.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adds an optional CURL-based HTTP/SSE networking backend with proxy
support, a new networking lib, and CI/release/testing to build and
verify both CURL and Boost.Beast variants.
> 
> - **Core networking**:
> - Introduce `libs/networking` with `CurlMultiManager` and CURL
integration (`launchdarkly::networking`).
> - Add `network::Requester` abstraction to switch between CURL and
Boost.Beast.
> - **SDK changes**:
> - Implement CURL SSE client and HTTP requester; wire into
client/server SDKs and internal events.
> - Add proxy config (`HttpProperties.Proxy`) and C bindings
(`*_HttpProperties_Proxy`); pass to SSE builder.
>   - Default/properties updated to carry `ProxyOptions`.
> - **Build/CMake**:
> - New option `LD_CURL_NETWORKING` to enable CURL networking;
`find_package(CURL QUIET)` in config.
>   - Add code coverage option `LD_BUILD_COVERAGE` and coverage script.
> - **CI/Release**:
> - New `install-curl` action; extend shared CI to toggle CURL builds
and simulate releases (Linux/macOS/Windows).
> - Update client/server/SSE workflows to run both Boost.Beast and CURL
jobs; add cmake-integration CURL jobs.
> - Release action now builds/zips additional `*-curl` artifacts (with
`-experimental` suffix for server).
> - **Examples/Docs/Tests**:
>   - Add proxy validation Docker-compose example.
>   - Update README and cmake-tests docs for CURL setup.
> - Add unit tests for CURL requester and SSE CurlClient; extend
contract tests to advertise proxy capability.
>   - Release manifest includes `libs/networking`.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
f916106. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Matthew M. Keeler <[email protected]>

Author: kinyoklion

.github/actions/ci/action.yml

@@ -29,6 +29,10 @@ inputs:
     description: 'Whether to run ./build-release-windows.sh for the CMake target'
     required: false
     default: 'false'
+  use_curl:
+    description: 'Whether to enable CURL networking (LD_CURL_NETWORKING=ON)'
+    required: false
+    default: 'false'
 
 runs:
   using: composite
@@ -44,22 +48,30 @@ runs:
     - name: Install OpenSSL
       uses: ./.github/actions/install-openssl
       id: install-openssl
+    - name: Install CURL
+      if: inputs.use_curl == 'true'
+      uses: ./.github/actions/install-curl
+      id: install-curl
     - name: Build Library
       shell: bash
-      run: ./scripts/build.sh ${{ inputs.cmake_target }} ON
+      run: ./scripts/build.sh ${{ inputs.cmake_target }} ON ${{ inputs.use_curl }}
       env:
         BOOST_ROOT: ${{ steps.install-boost.outputs.BOOST_ROOT }}
         Boost_DIR: ${{ steps.install-boost.outputs.Boost_DIR }}
         OPENSSL_ROOT_DIR: ${{ steps.install-openssl.outputs.OPENSSL_ROOT_DIR }}
+        CURL_ROOT: ${{ steps.install-curl.outputs.CURL_ROOT }}
+        CMAKE_PREFIX_PATH: ${{ steps.install-curl.outputs.CURL_ROOT }}
     - name: Build Tests
       id: build-tests
       if: inputs.run_tests == 'true'
       shell: bash
-      run: ./scripts/build.sh gtest_${{ inputs.cmake_target }} ON
+      run: ./scripts/build.sh gtest_${{ inputs.cmake_target }} ON ${{ inputs.use_curl }}
       env:
         BOOST_ROOT: ${{ steps.install-boost.outputs.BOOST_ROOT }}
         Boost_DIR: ${{ steps.install-boost.outputs.Boost_DIR }}
         OPENSSL_ROOT_DIR: ${{ steps.install-openssl.outputs.OPENSSL_ROOT_DIR }}
+        CURL_ROOT: ${{ steps.install-curl.outputs.CURL_ROOT }}
+        CMAKE_PREFIX_PATH: ${{ steps.install-curl.outputs.CURL_ROOT }}
     - name: Run Tests
       if: steps.build-tests.outcome == 'success'
       shell: bash
@@ -70,16 +82,30 @@ runs:
     - name: Simulate Release (Linux/MacOS)
       if: inputs.simulate_release == 'true'
       shell: bash
-      run: ./scripts/build-release.sh ${{ inputs.cmake_target }}
+      run: |
+        if [ "${{ inputs.use_curl }}" == "true" ]; then
+          ./scripts/build-release.sh ${{ inputs.cmake_target }} --with-curl
+        else
+          ./scripts/build-release.sh ${{ inputs.cmake_target }}
+        fi
       env:
         BOOST_ROOT: ${{ steps.install-boost.outputs.BOOST_ROOT }}
         OPENSSL_ROOT_DIR: ${{ steps.install-openssl.outputs.OPENSSL_ROOT_DIR }}
+        CURL_ROOT: ${{ steps.install-curl.outputs.CURL_ROOT }}
+        CMAKE_PREFIX_PATH: ${{ steps.install-curl.outputs.CURL_ROOT }}
 
     - name: Simulate Release (Windows)
       if: inputs.simulate_windows_release == 'true'
       shell: bash
-      run: ./scripts/build-release-windows.sh ${{ inputs.cmake_target }}
+      run: |
+        if [ "${{ inputs.use_curl }}" == "true" ]; then
+          ./scripts/build-release-windows.sh ${{ inputs.cmake_target }} --with-curl
+        else
+          ./scripts/build-release-windows.sh ${{ inputs.cmake_target }}
+        fi
       env:
         BOOST_ROOT: ${{ steps.install-boost.outputs.BOOST_ROOT }}
         OPENSSL_ROOT_DIR: ${{ steps.install-openssl.outputs.OPENSSL_ROOT_DIR }}
         Boost_DIR: 'C:\local\boost_1_87_0\lib64-msvc-14.3\cmake\Boost-1.87.0'
+        CURL_ROOT: ${{ steps.install-curl.outputs.CURL_ROOT }}
+        CMAKE_PREFIX_PATH: ${{ steps.install-curl.outputs.CURL_ROOT }}

.github/actions/cmake-test/action.yml

@@ -11,6 +11,10 @@ inputs:
   toolset:
     description: 'Boost toolset'
     required: false
+  cmake_extra_args:
+    description: 'Extra arguments to pass to CMake'
+    required: false
+    default: ''
 
 runs:
   using: composite
@@ -36,6 +40,7 @@ runs:
         BOOST_ROOT: ${{ steps.install-boost.outputs.BOOST_ROOT }}
         OPENSSL_ROOT_DIR: ${{ steps.install-openssl.outputs.OPENSSL_ROOT_DIR }}
         CMAKE_INSTALL_PREFIX: ../LAUNCHDARKLY_INSTALL
+        CMAKE_EXTRA_ARGS: ${{ inputs.cmake_extra_args }}
     - name: Build the SDK
       shell: bash
       run: |

.github/actions/install-curl/action.yml

@@ -0,0 +1,64 @@
+name: Install CURL
+description: 'Install CURL development libraries for all platforms.'
+
+outputs:
+  CURL_ROOT:
+    description: The location of the installed CURL.
+    value: ${{ steps.determine-root.outputs.CURL_ROOT }}
+
+runs:
+  using: composite
+  steps:
+    # Linux: Install via apt-get
+    - name: Install CURL for Ubuntu
+      if: runner.os == 'Linux'
+      id: apt-action
+      shell: bash
+      run: |
+        sudo apt-get update
+        sudo apt-get install -y libcurl4-openssl-dev
+        echo "CURL_ROOT=/usr" >> $GITHUB_OUTPUT
+
+    # macOS: Install via homebrew
+    - name: Install CURL for macOS
+      if: runner.os == 'macOS'
+      id: brew-action
+      shell: bash
+      run: |
+        brew install curl
+        echo "CURL_ROOT=$(brew --prefix curl)" >> $GITHUB_OUTPUT
+
+    # Windows: Build CURL from source with MSVC using helper script
+    - name: Install CURL for Windows
+      if: runner.os == 'Windows'
+      id: windows-action
+      shell: pwsh
+      run: |
+        # Use the build script from the repository
+        & "${{ github.workspace }}\scripts\build-curl-windows.ps1" -Version "8.11.1" -InstallPrefix "C:\curl-install"
+
+        if ($LASTEXITCODE -ne 0) {
+          Write-Error "CURL build failed"
+          exit 1
+        }
+
+        echo "CURL_ROOT=C:\curl-install" >> $env:GITHUB_OUTPUT
+
+    - name: Determine root
+      id: determine-root
+      shell: bash
+      run: |
+        if [ ! -z "$ROOT_APT" ]; then
+          echo "CURL_ROOT=$ROOT_APT" >> $GITHUB_OUTPUT
+          echo Setting CURL_ROOT to "$ROOT_APT"
+        elif [ ! -z "$ROOT_BREW" ]; then
+          echo "CURL_ROOT=$ROOT_BREW" >> $GITHUB_OUTPUT
+          echo Setting CURL_ROOT to "$ROOT_BREW"
+        elif [ ! -z "$ROOT_WINDOWS" ]; then
+          echo "CURL_ROOT=$ROOT_WINDOWS" >> $GITHUB_OUTPUT
+          echo Setting CURL_ROOT to "$ROOT_WINDOWS"
+        fi
+      env:
+        ROOT_APT: ${{ steps.apt-action.outputs.CURL_ROOT }}
+        ROOT_BREW: ${{ steps.brew-action.outputs.CURL_ROOT }}
+        ROOT_WINDOWS: ${{ steps.windows-action.outputs.CURL_ROOT }}