Add RequestType and ResponseType Metadata

[Jmoore1127]

Is your feature request related to a problem? Please describe.

I am attempting an upgrade of some tooling that was previously using grpc.load(...) to use the new proto-loader. The parsed proto the is returned is now much simpler and easier to use in many ways. However, the RequestType and ResponseType information is no longer available in proto-loader. The tooling I am working with uses this information in order to produce sample json objects for the user.

Describe the solution you'd like

I would like to add this metadata to the method definition returned from createMethodDefinition in grpc-protobufjs/src/index.ts so that the tooling has access to this information.

Additional context

I have submitted a PR with a basic implementation.