Include the time of the create event in Spaces Summary. (#9928)

This is an update based on changes to MSC2946. The origin_server_ts
of the m.room.create event is copied into the creation_ts field for each
room returned from the spaces summary.

changelog.d/9928.bugfix

@@ -0,0 +1 @@
+Include the `origin_server_ts` property in the experimental [MSC2946](https://github.com/matrix-org/matrix-doc/pull/2946) support to allow clients to properly sort rooms.

synapse/handlers/space_summary.py

@@ -347,6 +347,7 @@ class SpaceSummaryHandler:
                 stats["history_visibility"] == HistoryVisibility.WORLD_READABLE
             ),
             "guest_can_join": stats["guest_access"] == "can_join",
+            "creation_ts": create_event.origin_server_ts,
             "room_type": room_type,
         }