[Persistence] Add support for pinned messages

Signed-off-by: Sahil Kumar <[email protected]>

packages/stream_chat_persistence/lib/src/entity/pinned_messages.dart

@@ -0,0 +1,7 @@
+import 'package:moor/moor.dart';
+
+import 'messages.dart';
+
+/// Represents a [PinnedMessages] table in [MoorChatDatabase].
+@DataClassName('PinnedMessageEntity')
+class PinnedMessages extends Messages {}