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/user_directory.py

@@ -114,7 +114,7 @@ class UserDirectoryHandler(StateDeltasHandler):
         if self._is_processing:
             return
 
-        async def process():
+        async def process() -> None:
             try:
                 await self._unsafe_process()
             finally: