Make `grpclib` dependency optional

First off, thank you for this tool, I was barely able to work with the Python code that protoc generates by default.

I believe that for my usecase, the `grpclib` dependency (and the indirect dependency on four other packages) is unnecessary, and it might be worth thinking about making it optional, and enabling it via an extra. I assume this would be a breaking change, since I don't think you can make extras enabled by default. But maybe it's an idea for v2 or a future major release.

Basically all I need is to decode messages arriving from a socket. For that I don't need more than what is imported in the generated files: `Message`, `Enum`, and the `*_field` functions. As far as I can tell, these do not depend on `grpclib`.