Require type hints in the handlers module. (#10831)

Adds missing type hints to methods in the synapse.handlers
module and requires all methods to have type hints there.

This also removes the unused construct_auth_difference method
from the FederationHandler.

synapse/handlers/register.py

@@ -125,7 +125,7 @@ class RegistrationHandler(BaseHandler):
         localpart: str,
         guest_access_token: Optional[str] = None,
         assigned_user_id: Optional[str] = None,
-    ):
+    ) -> None:
         if types.contains_invalid_mxid_characters(localpart):
             raise SynapseError(
                 400,