Currently running the codegen for C# is a manual step. We should include an MS Build task in the NuGet package if we can. That will require some manual intervention to use in project files, but it's better than nothing.

Ccs:
@igorgatis, @csmith, @mmdriley who have been interested in making the experience better

@kkm000 and @jtattermusch who have been thinking about this for GRPC: grpc/grpc#4805

Advice welcome :)