Support icons for Identity Providers (#9154)

changelog.d/9154.feature

docs/sample_config.yaml

mypy.ini

synapse/config/oidc_config.py

synapse/config/server.py

synapse/federation/federation_server.py

synapse/federation/transport/server.py

synapse/handlers/cas_handler.py

synapse/handlers/oidc_handler.py

synapse/handlers/room.py

synapse/handlers/saml_handler.py

synapse/handlers/sso.py

synapse/http/endpoint.py

synapse/res/templates/sso_login_idp_picker.html

synapse/rest/client/v1/room.py

synapse/storage/databases/main/room.py

synapse/types.py

synapse/util/stringutils.py

tests/http/test_endpoint.py