Refactor HomeserverConfig so it can be typechecked (#6137)

synapse/config/ratelimiting.py

synapse/config/registration.py

synapse/config/repository.py

synapse/config/room_directory.py

synapse/config/saml2_config.py

synapse/config/server.py

synapse/config/server_notices_config.py

synapse/config/spam_checker.py

synapse/config/stats.py

synapse/config/third_party_event_rules.py

synapse/config/tls.py

synapse/config/tracer.py

synapse/config/user_directory.py

synapse/config/voip.py

synapse/config/workers.py

tests/config/test_tls.py

tox.ini