Add EntityStorageControllerInterface::getClass()

In order to more easily use different classes for the same entity type, add a EntityStorageControllerInterface::getClass() method that returns the class name. The entity storage controller base classes can default to the class name from the entity info. More complex storage controllers can add their own logic, without having to override the create() and buildQuery() completely. In order to dynamically determine the class to use, the getClass() method should receive the raw entity data.