Support the stable /hierarchy endpoint from MSC2946 (#11329)

This also makes additional updates where the implementation
had drifted from the approved MSC.

Unstable endpoints will be removed at a later data.

synapse/federation/transport/client.py

@@ -1192,10 +1192,24 @@ class TransportLayerClient:
         )
 
     async def get_room_hierarchy(
-        self,
-        destination: str,
-        room_id: str,
-        suggested_only: bool,
+        self, destination: str, room_id: str, suggested_only: bool
+    ) -> JsonDict:
+        """
+        Args:
+            destination: The remote server
+            room_id: The room ID to ask about.
+            suggested_only: if True, only suggested rooms will be returned
+        """
+        path = _create_v1_path("/hierarchy/%s", room_id)
+
+        return await self.client.get_json(
+            destination=destination,
+            path=path,
+            args={"suggested_only": "true" if suggested_only else "false"},
+        )
+
+    async def get_room_hierarchy_unstable(
+        self, destination: str, room_id: str, suggested_only: bool
     ) -> JsonDict:
         """
         Args: