Convert internal pusher dicts to attrs classes. (#8940)

changelog.d/8940.misc

mypy.ini

synapse/push/__init__.py

synapse/push/emailpusher.py

synapse/push/httppusher.py

synapse/push/pusher.py

synapse/push/pusherpool.py

synapse/replication/slave/storage/_slaved_id_tracker.py

synapse/replication/slave/storage/pushers.py

synapse/rest/admin/users.py

synapse/rest/client/v1/pusher.py

synapse/storage/databases/main/__init__.py

synapse/storage/databases/main/pusher.py

synapse/storage/util/id_generators.py

tests/push/test_email.py

tests/push/test_http.py

tests/rest/admin/test_user.py