Expose tombstone status in room details (#19737)

Exposes `tombstoned` and `replacement_room` in room details on admin API
endpoint `GET /_synapse/admin/v1/rooms/<room_id>`. Resolves #18347

docs/admin_api/rooms.md

@@ -308,6 +308,9 @@ The following fields are possible in the JSON response body:
   If the room does not define a type, the value will be `null`.
 * `forgotten` - Whether all local users have
   [forgotten](https://spec.matrix.org/latest/client-server-api/#leaving-rooms) the room.
+* `tombstoned` - Whether the room has been tombstoned (permanently closed).
+* `replacement_room` - The room ID of the new room that users should join instead, if this room was tombstoned. Will be 
+  `null` if the room has not been tombstoned, or if it was tombstoned without designating a successor room.
 
 The API is:
 
@@ -337,7 +340,9 @@ A response body like the following is returned:
   "history_visibility": "shared",
   "state_events": 93534,
   "room_type": "m.space",
-  "forgotten": false
+  "forgotten": false,
+  "tombstoned": false,
+  "replacement_room": null
 }
 ```