Do not check for EntityAdapter class explicitly in content entity datasource

The content entity datasource currently checks explicitly for the EntityAdapter class in several occasions where it receives a ComplexDataInterface object and wants to make sure getValue() will return an entity.

However, I'm pretty sure the class is an implementation detail and in no way "stable" or "guaranteed", so it would be better to find an alternative.
One easy solution would be to just call getValue() first and then check whether that value implements EntityInterface – a much cleaner and more direct check.

Or, with just slightly more work, we could add a method getEntity() which returns either an EntityInterface or NULL and does one or more checks for this. E.g., with this we could very easily catch the case that the ComplexDataInterface object could itself implement EntityInterface.