feat: cache archived rooms to access them with `getRoomById`

getRoomById searches now in the local cache for the given room and returns null if not
found. If you have loaded the [archive] before, it can also return
archived rooms.
This should make it much easier to display
archived rooms in the client.

lib/src/client.dart

@@ -275,10 +275,14 @@ class Client extends MatrixApi {
     return null;
   }
 
+  /// Searches in the local cache for the given room and returns null if not
+  /// found. If you have loaded the [archive] before, it can also return
+  /// archived rooms.
   Room getRoomById(String id) {
-    for (final room in rooms) {
+    for (final room in <Room>[...rooms, ..._archivedRooms]) {
       if (room.id == id) return room;
     }
+
     return null;
   }
 
@@ -645,8 +649,10 @@ class Client extends MatrixApi {
         avatarUrl: profile.avatarUrl);
   }
 
+  final List<Room> _archivedRooms = [];
+
   Future<List<Room>> get archive async {
-    final archiveList = <Room>[];
+    _archivedRooms.clear();
     final syncResp = await sync(
       filter: '{"room":{"include_leave":true,"timeline":{"limit":10}}}',
       timeout: 0,
@@ -681,10 +687,10 @@ class Client extends MatrixApi {
             ));
           }
         }
-        archiveList.add(leftRoom);
+        _archivedRooms.add(leftRoom);
       }
     }
-    return archiveList;
+    return _archivedRooms;
   }
 
   /// Uploads a file and automatically caches it in the database, if it is small enough