Add type hints to various handlers. (#9223)

changelog.d/9223.misc

mypy.ini

synapse/handlers/acme.py

synapse/handlers/acme_issuing_service.py

synapse/handlers/groups_local.py

synapse/handlers/search.py

synapse/handlers/set_password.py

synapse/handlers/state_deltas.py

synapse/handlers/stats.py

synapse/handlers/typing.py

synapse/handlers/user_directory.py

synapse/storage/databases/main/search.py

synapse/storage/databases/main/stats.py

synapse/storage/databases/main/user_directory.py