Add missing type hints to config classes. (#12402)

changelog.d/12402.misc

mypy.ini

synapse/_scripts/review_recent_signups.py

synapse/app/_base.py

synapse/config/_base.py

synapse/config/_base.pyi

synapse/config/account_validity.py

synapse/config/api.py

synapse/config/appservice.py

synapse/config/auth.py

synapse/config/background_updates.py

synapse/config/cache.py

synapse/config/captcha.py

synapse/config/cas.py

synapse/config/consent.py

synapse/config/database.py

synapse/config/emailconfig.py

synapse/config/experimental.py

synapse/config/federation.py

synapse/config/groups.py