Bazel rules for C#

[PolarBear292208]

What language does this apply to?
C#

Describe the problem you are trying to solve.
There are no standard Bazel rules to generate C# source code for protocol buffers.

Describe the solution you'd like
A csharp_proto_library to be included as standard as part of @protobuf//bazel

Describe alternatives you've considered

• I tried to make sense of the C++/Java/Python rule definitions, but couldn't figure out how to modify them to get protoc to output C# files
• I also looked at the following alternative proto rules, but similarly couldn't figure out how to add C# support:
  • https://github.com/stackb/rules_proto
  • https://github.com/rules-proto-grpc/rules_proto_grpc

Additional context
None

[jskeet]

This isn't something I would have the time or expertise to do, I'm afraid. (We do have some Bazel rules for C# in the GAPIC generator, but I strongly suspect they're not ready for general-purpose protobuf usage.)

Assigning back to Jason for next steps.