Fix raw response with selections on abstract type

[tobias-tengler]

Previously all selections done on abstract types, would be generated as required properties in the raw response type. This PR tracks all selections on abstract types separately and only makes them required if the concrete type implements the abstract type. The change in the generated types can be seen in this diff.

To enable a gradual rollout, a disable_more_precise_abstract_selection_raw_response_type feature flag has been added that allows to opt operations out of the new raw response type generation.