[Feat]: Implement remaining methods of the push notifications API

[ishymko]

Push notifications related methods are not implemented fully across client and server:

1. "not implemented" is thrown in rest_handler.py for list_push_notifications.
2. rest_adapter.py does not declare delete route.
3. list and delete are not declared in the base_client.py
4. list and delete are not declared in the client/base.py.
5. (Optional): client uses callback terminology for methods and push notifications for data model (as data model is generated from the proto) which can be confusing and 1.0 is a good chance to fix it.