Allow denying or shadow banning registrations via the spam checker (#8034)

changelog.d/8034.feature

synapse/events/spamcheck.py

synapse/handlers/auth.py

synapse/handlers/cas_handler.py

synapse/handlers/oidc_handler.py

synapse/handlers/register.py

synapse/handlers/saml_handler.py

synapse/rest/client/v2_alpha/register.py

synapse/spam_checker_api/__init__.py

.../databases/main/schema/delta/58/07persist_ui_auth_ips.sql

synapse/storage/databases/main/ui_auth.py

tests/handlers/test_oidc.py

tests/handlers/test_register.py

tests/handlers/test_user_directory.py