add proto descriptor set as source of method definitions

[jizhilong]

protocol buffer descriptor set is a special type of protocol buffer message-it is contains a description of all message/grpc-service definitions compiled by protoc.

For users working with a bunch of .proto files, it's much more practical to supply grpc definitions with a single descriptoset file, instead of a directory of .proto files. And other grpc related tool like grpccurl is able to read grpc service definitions from a descriptor set file too, with a cli option --protoset.

@rmedvedev I will be happy to contribute a PR for this feature if you are ok with that.