Integrate knock rooms with the public rooms directory (#9359)

This PR implements the ["Changes regarding the Public Rooms Directory"](https://github.com/Sorunome/matrix-doc/blob/soru/knock/proposals/2403-knock.md#changes-regarding-the-public-rooms-directory) section of knocking MSC2403.

Specifically, it:

* Allows rooms with `join_rule` "knock" to be returned by the query behind the public rooms directory
* Adds the field `join_rule` to each room entry returned by a public rooms directory query, so clients can know whether to attempt a join or knock on a room

Based on https://github.com/matrix-org/synapse/issues/6739. Complement tests for this change: https://github.com/matrix-org/complement/pull/72

synapse/handlers/room_list.py

@@ -169,6 +169,7 @@ class RoomListHandler(BaseHandler):
                 "world_readable": room["history_visibility"]
                 == HistoryVisibility.WORLD_READABLE,
                 "guest_can_join": room["guest_access"] == "can_join",
+                "join_rule": room["join_rules"],
             }
 
             # Filter out Nones – rather omit the field altogether