Provide a dynamic return type for getStorage on the entity type manager

[mglaman]

We can use a dynamic return type on getStorage to do a proper type hint on known entity storages.

[mglaman]

Since it's impossible to read the entity annotations and discern what entity storage class to return, we can use mapping in the parameters. This allows each project to customize custom entity storage definitions.

Something like:

	drupal:
		entityTypeStorageMapping:
			node: Drupal\node\NodeStorage
			taxonomy_term: Drupal\taxonomy\TermStorage
			user: Drupal\user\UserStorage