confighttp: add support for binding unix sockets

[axw]

Description

Embed confignet.AddrConfig in confighttp.ServerConfig, replacing the Endpoint field. This introduces the "transport" config attribute, which can be set to "unix" for binding a Unix domain socket.

Link to tracking issue

Fixes #8752
Fixes #14187

Testing

$ ./bin/otelcorecol_linux_amd64 \
    --config examples/local/otel-config.yaml \
    --set receivers::otlp::protocols::http::transport=unix \
    --set receivers::otlp::protocols::http::endpoint=\"@otlphttp\"

$ curl --abstract-unix-socket otlphttp http://testing.invalid:1234/v1/traces
405 method not allowed, supported: [POST]

Documentation

Updated README.md

[codspeed-hq]

CodSpeed Performance Report

Merging #14248 will degrade performances by 77.31%

_{Comparing axw:confighttp-addrconfig (fd31906) with main (a0cbea7)}

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

Summary

❌ 1 regression
✅ 72 untouched

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	zstdWithConcurrency	6.9 µs	30.3 µs	-77.31%

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.17%. Comparing base (152042e) to head (fd31906).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14248      +/-   ##
==========================================
- Coverage   92.18%   92.17%   -0.02%     
==========================================
  Files         668      668              
  Lines       41463    41473      +10     
==========================================
+ Hits        38223    38228       +5     
- Misses       2209     2212       +3     
- Partials     1031     1033       +2     

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[axw]

This is a breaking change for the API, but not for user configuration. Naturally there are a bunch of compilation errors in contrib, which I can fix if/when this is accepted.