Propose Refactor: Add getIdentity() to Entity\UserInterface and findByIdentity() to the Mapper\UserInterface

What the PR contains
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Currently the modules works out which fields to use as the identity outside of the mapper & entity class. By moving these into the entity & mapper classes they can more easily be redefined if needed by module extending ZfcUser.

Issues
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Modules which currently extend/implement their own user entity and or user mapper would need to be updated should this PR be accepted.

ZfcUserDoctrineORM being one, I have included a fix here for that module too here: