[REQ] Add interface the represents all InlineResponses generated

Is your feature request related to a problem? Please describe.

Right now in a project I'm working on I have a class that does the calls, so it gets a ModelInterface and returns a ModelInterface which is the Hydramember.
Then the code inspector complains that I call methods like getHydratotalItems() and getHydramember() on a ModelInterface object.

I think those methods should be described in an interface so they don't have to be redefined in all InlineResponse, most of the stuff there could even be moved to a base class so we have less code duplication.
And also the we can pass all those classes to a method to handle calls like I'm trying to do.

Describe the solution you'd like

Add an interface that defines the InlineResponse objects and maybe a base class for them also.

Describe alternatives you've considered

Just adding the interface.

Additional context

Non vengono indicati file, test o punti di ingresso del generatore. Inizia individuando le implementazioni Java di InlineResponse e i template di generazione che le producono, quindi determina i metodi condivisi e l’ambito di un’eventuale classe base; il lavoro è completato quando i tipi InlineResponse generati condividono l’interfaccia richiesta e i chiamanti possono usare quei metodi senza ridefinirli.