[Java client] add a way to duplicate an model class

Created by: jmini

There are scenario where being able to create a new instance of a model class with the same values as an other one is necessary.

Example:

Pet pet = api.getPetById().petIdPath(id).executeAs(r -> r.thenReturn());

Pet body = pet; // this should be a new instance of Pet, a copy with the same values.
pet.setName("New name");
api.updatePet().body(body).execute(r -> r.prettyPeek());

Pet pet2 = api.getPetById().petIdPath(id).executeAs(r -> r.thenReturn());

// here you need the pet instance as it was (without any modification) 
// and pet2 to perform some assertions

I suggest to add two constructors to the models classes:

• An empty one (current behaviour)
• A constructor accepting an other instance of the same class

In the second case, the values will be copied.