Use CMake's protobuf module

[Ryanf55]

Relates to gazebosim/gz-cmake#9

Summary

This is removing the need for the custom find script for protobuf.

Test it

Compile on CI

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[mjcarroll]

I don't think that this should be needed, as the top-level CMakeLists file should already be finding protobuf, correct?

[Ryanf55]

That what I would have thought, but it failed build on my system. I didn't find the call to add_subdirectory(src) so I'm a bit lost how the CMakeLists is working in the first place.

[scpeters]

as noted by @traversaro in gazebosim/gz-cmake#363 (comment)

Unfortunatly at the moment the FindProtobuf.cmake shipped with CMake does not work correctly with protobuf >= 23.2

This is the cause of the macOS CI failure in this job:

• https://build.osrfoundation.org/job/ignition_msgs-ci-pr_any-homebrew-amd64/1282/

CMake Warning at /usr/local/Cellar/cmake/3.26.4/share/cmake/Modules/FindProtobuf.cmake:524 (message):
  Protobuf compiler version 23.4 doesn't match library version 4.23.4
Call Stack (most recent call first):
  CMakeLists.txt:85 (find_package)


-- Found Protobuf: /usr/local/lib/libprotobuf.dylib (found version "4.23.4") 
...
[  2%] Linking CXX executable ../bin/gz_msgs_gen
Undefined symbols for architecture x86_64:
  "absl::lts_20230125::hash_internal::MixingHashState::LowLevelHashImpl(unsigned char const*, unsigned long)", referenced from:
      absl::lts_20230125::hash_internal::MixingHashState::combine_contiguous(absl::lts_20230125::hash_internal::MixingHashState, unsigned char const*, unsigned long) in Generator.cc.o
...

[Ryanf55]

Closing for now to reduce the noise. It can be revisited when CMake+Protobuf work out of the box nicely again.