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/room_summary.py

@@ -541,7 +541,7 @@ class RoomSummaryHandler:
         origin: str,
         requested_room_id: str,
         suggested_only: bool,
-    ):
+    ) -> JsonDict:
         """
         Implementation of the room hierarchy Federation API.