[StreamChatPersistence] Add initial implementation

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

packages/stream_chat_persistence/lib/src/converter/map_converter.dart

@@ -0,0 +1,22 @@
+import 'dart:convert';
+
+import 'package:moor/moor.dart';
+
+///
+class MapConverter<T> extends TypeConverter<Map<String, T>, String> {
+  @override
+  Map<String, T> mapToDart(fromDb) {
+    if (fromDb == null) {
+      return null;
+    }
+    return Map<String, T>.from(jsonDecode(fromDb) ?? {});
+  }
+
+  @override
+  String mapToSql(value) {
+    if (value == null) {
+      return null;
+    }
+    return jsonEncode(value);
+  }
+}