Add support for stable MSC2858 API (#9617)

The stable format uses different brand identifiers, so we need to support two
identifiers for each IdP.

synapse/handlers/sso.py

@@ -98,6 +98,11 @@ class SsoIdentityProvider(Protocol):
         """Optional branding identifier"""
         return None
 
+    @property
+    def unstable_idp_brand(self) -> Optional[str]:
+        """Optional brand identifier for the unstable API (see MSC2858)."""
+        return None
+
     @abc.abstractmethod
     async def handle_redirect_request(
         self,