cmd/protoc-gen-go-grpc: add hooks to allow overriding generated code (#5056)
cmd/protoc-gen-go-grpc: add hooks to allow overriding generated code (#5056)


stanley-cheung/grpc-go,dfawley/grpc-go,stanley-cheung/grpc-go,grpc/grpc-go,dfawley/grpc-go,menghanl/grpc-go,menghanl/grpc-go,grpc/grpc-go