Allow guests to send state events (#11378)

changelog.d/11378.feature

+Allow guests to send state events per [MSC3419](https://github.com/matrix-org/matrix-doc/pull/3419).
\ No newline at end of file

synapse/rest/client/room.py

@@ -187,7 +187,7 @@ class RoomStateEventRestServlet(TransactionRestServlet):
         state_key: str,
         txn_id: Optional[str] = None,
     ) -> Tuple[int, JsonDict]:
-        requester = await self.auth.get_user_by_req(request)
+        requester = await self.auth.get_user_by_req(request, allow_guest=True)
 
         if txn_id:
             set_tag("txn_id", txn_id)