Add plugin integration for sbt

[jvican]

@borissmidt Do you plan on integrating monix-grpc with sbt? If so we might want to build a quick sbt plugin for monix-grpc just like fs2-grpc has so that our users can quickly configure it. If this is something you want to take on, that'd be great.

[borissmidt]

I think we can just do like the zio grpc project so there is no more need for a custom plugin:
https://github.com/scalapb/zio-grpc
Or do you think making a hard dependency in the pom to the scalapb plugin and configuring it is a better idea?