Make Protobuf support extensible for gRPC

Spring gRPC needs to replicate `ProtobufHttpMessageConverter` and `ProtobufHttpMessageWriter` in order to deal with trailers and other concerns, see the current [wip branch](https://github.com/scratches/grpc-reflect/tree/0688ce6b3f7a233be3da8e8e6bd8b2c706fcf550/lib/src/main/java/org/springframework/grpc). We might be able to make some adjustments to make them more friendly for extension.