Method type specific MethodDescriptor wrapper

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

It would be great if MethodDescriptors would be wrapped in method type specific class/interface, similar to RequestMethods. It would enable gRPC tooling to know what type of method it is at compile type, so it would be possible to return appropriate action interfaces based on the method type (does method have single/multiple request/responses). This would help to reduce the tools API verbosity and a bit of complexity.

### Describe alternatives you've considered
Current approach is to have differently named methods (unaryMethod(), serverStreamingMethod(), ...) and check if appropriate method type is passed at runtime.

### Additional context

I'm trying to create a gRPC method mocking tool for easier testing - [gRPCMock](https://github.com/Fadelis/grpcmock/). This feature would help determining what type of stub needs to be returned and have single name method for all types.

まず既存の MethodDescriptor API と RequestMethods API を確認し、次に grpc-java ツールが現在どのようにメソッド型を区別しているかを追跡します。メソッド固有のラッパーの範囲と、それらが提供すべきコンパイル時の動作を定義します。API が実行時チェックに依存せず、4 つのリクエスト/レスポンスメソッド形状をサポートすれば完了です。

索引モデルが issue の本文から書いたものです。