Improve type safety for command, forward and effect handlers

[marcellanz]

The Go gRPC protobuf compiler plugin doesn't expose service descriptors. As a consequence, command, forward and effect handlers require the user to provide unchecked plain strings to reference a commands gRPC service name or services method names. The same applies for service names and command names used with effects and forwards.

Provide an idiomatic way to improve type safety in this regard.

[marcellanz]

It seems to be something is happening to export service descriptors: grpc/grpc-go#3762 (comment)

[marcellanz]

Service descriptors are now exported: grpc/grpc-go#4035