…Api for stubs in all transports

This PR essentially does the following:
1) Cleans `@BetaApi("A restructuring of stub classes is planned, so this may break in the future")` annotaiton for Stub-related methods and Stub classes themselves, because those methods and classes have been in "beta" state like that for several  years already and are de-facto GA.
2) Makes sure that all HttpJson (REST) related classes and methods on the surface of the client are marked as beta for `grpc+rest` (mixed transport) clients. This is necessary in the context of the upcoming REGAPIC rollout to indicate that the newly released REST transport funcitonality is released at Beta quality level.