feat: make data classes serializable (#2223) (#2241)

...so class instances will be shown as `Type(a: xx, b: yy)` instead of
`Instance 'Type'` when parsed by readables (e.g. console).

Co-authored-by: Alex Li <[email protected]>

Author: peilinok

lib/src/_serializable.dart

@@ -0,0 +1,6 @@
+abstract class AgoraSerializable {
+  Map<String, dynamic> toJson();
+
+  @override
+  String toString() => '$runtimeType({$toJson()})';
+}