google flatbuffer java support feature request

While it has been discussed, I don't think there is an explicit feature request for grpc-java support of [google flatbuffers](https://github.com/google/flatbuffers), maintained by @gwvo, as a replacement for protobuf messages, initially integrated into the grpc c at https://github.com/grpc/grpc/issues/5438 and https://github.com/google/flatbuffers/commit/48f37f9e0a04f2b60046dda7fef20a8b0ebc1a70.

Use cases in grpc-java have also been mentioned in https://github.com/grpc/grpc-java/issues/2139 and https://github.com/grpc/grpc-java/issues/1403.