Bazel Rule Support for Kotlin

[jamesward]

To support generating Kotlin protos via Bazel, we need some support for the classes which the generated Kotlin protos use:
https://github.com/protocolbuffers/protobuf/tree/master/java/kotlin

Can be inspired by: https://github.com/protocolbuffers/protobuf/blob/master/java/core/BUILD

This will be used by new rules which will have examples:
grpc/grpc-kotlin#266

Please assign to: @deannagarcia

[deannagarcia]

This has been resolved in the grpc-kotlin repository here: grpc/grpc-kotlin#290

[Kernald]

Shouldn't this ultimately belong to this repo? It feels weird having rules for protobuf, which shouldn't require gRPC, in the gRPC repo.

[deannagarcia]

This is a temporary solution to unblock the gRPC team to use kotlin proto examples, a long term solution will be coming later that will ultimately belong to this repo.

[SanjayVas]

I think this should stay open, as what's in grpc-kotlin is a workaround. Note that this issue isn't for Kotlin protobuf Bazel rules themselves, but rather the fact that such rules depend on protobuf-kotlin. Currently, the way to get this dependency is as a pre-built Maven artifact (com.google.protobuf:protobuf-kotlin), whereas it would be cleaner for Bazel projects to be able to build this artifact from source.