Add service descriptor to code generator

[Gorzen]

This PR adds service descriptor in the code generator for gRPC services.

The service descriptor is useful to get general information about the service. For instance, in our case, we use the name to load the service's configuration.

[lomigmegard]

Hi @ahjohannessen ,

Would you have time to have a look at this PR? It exposes the service descriptor to the generated gRPC service companion objects.

There could be alternative implementation or naming, but the core idea of linking the GeneratedCompanion to its corresponding descriptor (Java or ScalaPB one) would be very useful to build generic tooling around.

[ahjohannessen]

please fix mima by adding ProblemFilters.exclude[ReversedMissingMethodProblem]("fs2.grpc.GeneratedCompanion.serviceDescriptor") to build.sbt

[Gorzen]

Thank you! @ahjohannessen

[ahjohannessen]

@Gorzen np - I have made a new release