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`.

L’issue nomme les fichiers générés ainsi que leurs imports Message, Enum et *_field ; examine d’abord ces points d’entrée, puis retrace où grpclib est déclaré et requis. Le travail est terminé lorsque le chemin de décodage uniquement ne requiert plus grpclib, tandis que la prise en charge de gRPC reste explicitement disponible, avec le comportement des dépendances vérifié.

Rédigé par le modèle d'indexation à partir du texte de l'issue.