Use a jar-packaged protoc instead requiring pre installed

[johanandren]

I've just stumbled on having the wrong protoc version installed for the Nth time. It would be nice if we could get rid of that as an external dependency of the build (when changing the files) and instead use a jar-packaged set version like how Akka gRPC does it.

I wouldn't want to change the way we do it with generating explicitly and then checking in the generated sources, only that we'd automagically have the right protoc version pulled in by the build.