Clean up classes and interfaces for native queries

[DavideD]

I think there's is something wrong in the design of the Mutiny and Stage sessions when it comes to native queries.

For example, this Mutiny.Session#createNativeQuery returns SelectionQuery. But, it should return a ReactiveNativeQueryImplementor like in ORM.

I don't think there is a way for users to access the methods in ReactiveNativeQuery at the moment. We need to add more testing.

(Noticed while adapting a test for HHH-18629)