Support fetching the spaces summary via GET over federation. (#9947)

Per changes in MSC2946, the C-S and S-S APIs for spaces summary
should use GET requests.

Until this is stable, the POST endpoints still exist.

This does not switch federation requests to use the GET version yet
since it is newly added and already deployed servers might not support
it. When switching to the stable endpoint we should switch to GET
requests.

changelog.d/9947.feature

+Update support for [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946): Spaces Summary.

synapse/federation/transport/client.py

@@ -995,6 +995,7 @@ class TransportLayerClient:
                returned per space
             exclude_rooms: a list of any rooms we can skip
         """
+        # TODO When switching to the stable endpoint, use GET instead of POST.
         path = _create_path(
             FEDERATION_UNSTABLE_PREFIX, "/org.matrix.msc2946/spaces/%s", room_id
         )

synapse/federation/transport/server.py

@@ -1376,6 +1376,32 @@ class FederationSpaceSummaryServlet(BaseFederationServlet):
     PREFIX = FEDERATION_UNSTABLE_PREFIX + "/org.matrix.msc2946"
     PATH = "/spaces/(?P<room_id>[^/]*)"
 
+    async def on_GET(
+        self,
+        origin: str,
+        content: JsonDict,
+        query: Mapping[bytes, Sequence[bytes]],
+        room_id: str,
+    ) -> Tuple[int, JsonDict]:
+        suggested_only = parse_boolean_from_args(query, "suggested_only", default=False)
+        max_rooms_per_space = parse_integer_from_args(query, "max_rooms_per_space")
+
+        exclude_rooms = []
+        if b"exclude_rooms" in query:
+            try:
+                exclude_rooms = [
+                    room_id.decode("ascii") for room_id in query[b"exclude_rooms"]
+                ]
+            except Exception:
+                raise SynapseError(
+                    400, "Bad query parameter for exclude_rooms", Codes.INVALID_PARAM
+                )
+
+        return 200, await self.handler.federation_space_summary(
+            room_id, suggested_only, max_rooms_per_space, exclude_rooms
+        )
+
+    # TODO When switching to the stable endpoint, remove the POST handler.
     async def on_POST(
         self,
         origin: str,

synapse/rest/client/v1/room.py

@@ -1020,6 +1020,7 @@ class RoomSpaceSummaryRestServlet(RestServlet):
             max_rooms_per_space=parse_integer(request, "max_rooms_per_space"),
         )
 
+    # TODO When switching to the stable endpoint, remove the POST handler.
     async def on_POST(
         self, request: SynapseRequest, room_id: str
     ) -> Tuple[int, JsonDict]: