[StreamChatPersistence] Add initial implementation

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

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

@@ -0,0 +1,25 @@
+import 'package:moor/moor.dart';
+import 'package:stream_chat_persistence/src/converter/map_converter.dart';
+
+@DataClassName('ReactionEntity')
+class Reactions extends Table {
+  TextColumn get userId => text()();
+
+  TextColumn get messageId =>
+      text().customConstraint('REFERENCES messages(id) ON DELETE CASCADE')();
+
+  TextColumn get type => text()();
+
+  DateTimeColumn get createdAt => dateTime()();
+
+  IntColumn get score => integer().nullable()();
+
+  TextColumn get extraData => text().nullable().map(MapConverter<Object>())();
+
+  @override
+  Set<Column> get primaryKey => {
+        messageId,
+        type,
+        userId,
+      };
+}