Add option to generate only gRPC service and method descriptors.

[notxcain]

Hey!

We use ZIO-based gRPC interfaces and non-ZIO versions create redundant generation and compilation overhead. I can work on this one. Just wanted to align with you in case there are any concerns or similar plans in that area.

[thesamet]

Hi @notxcain , yes, this would be something that can be accepted.