Allow specifying output directory for autogenerated files

[egilll]

• Useful for when you want to commit these files to source control, either in order to be able to track differences in the generated structs, or in order to allow a project to be opened and worked on without first invoking a build step.
• set CACHE STRING is used instead of option since otherwise IDEs will treat it as a bool.
• Minimum CMake version for the CMake helper function cmake_path is version 3.20, which was released in 2021, and so should hopefully be all right to use as a minimum version.

[tonda-kriz]

Nice, but I think that the SPB_PROTO_OUTPUT_DIR should be a parameter to the spb_protobuf_generate function and not an option, similar to https://cmake.org/cmake/help/latest/module/FindProtobuf.html#command:protobuf_generate.