Enumeration MethodKind

MethodKind represents the four method types that can be declared in protobuf with the stream keyword:

1. Unary: rpc (Input) returns (Output)
2. ServerStreaming: rpc (Input) returns (stream Output)
3. ClientStreaming: rpc (stream Input) returns (Output)
4. BiDiStreaming: rpc (stream Input) returns (stream Output)