Split up the protoc plugin to make gRPC content configurable

Currently there is a single plugin with limited support for generating gRPC client code, and there is demand to cover servicer generation #19, with grpclib or grpcio, or none #44 #20 .

Since AFAIK there is not standard way to provide arguments to protoc plugins, I propose that the plugin be exposed as multiple distinct executables, one to generate just the protobuf dataclasses, one to generate the dataclasses and grpclib client/server and one to generate dataclasses and grpcio client/server.

Are there other approaches worth considering?