Add types to synapse.util. (#10601)

changelog.d/10601.misc

mypy.ini

stubs/txredisapi.pyi

synapse/api/ratelimiting.py

synapse/config/ratelimiting.py

synapse/federation/sender/__init__.py

synapse/handlers/account_validity.py

synapse/handlers/appservice.py

synapse/handlers/presence.py

synapse/handlers/typing.py

synapse/rest/client/register.py

synapse/rest/synapse/client/new_user_consent.py

synapse/rest/synapse/client/pick_username.py

synapse/storage/databases/main/registration.py

synapse/types.py

synapse/util/__init__.py

synapse/util/async_helpers.py

synapse/util/batching_queue.py

synapse/util/caches/__init__.py

synapse/util/caches/deferred_cache.py