Refactor HomeserverConfig so it can be typechecked (#6137)

changelog.d/6137.misc

mypy.ini

synapse/config/_base.py

synapse/config/_base.pyi

synapse/config/api.py

synapse/config/appservice.py

synapse/config/captcha.py

synapse/config/cas.py

synapse/config/consent_config.py

synapse/config/database.py

synapse/config/emailconfig.py

synapse/config/groups.py

synapse/config/homeserver.py

synapse/config/jwt_config.py

synapse/config/key.py

synapse/config/logger.py

synapse/config/metrics.py

synapse/config/password.py

synapse/config/password_auth_providers.py

synapse/config/push.py