如何在flutter中解析动态变化的键的json数据？

dependencies:
  flutter:
    sdk: flutter
  json_annotation: ^4.0.1
  json_serializable: ^4.1.4

dev_dependencies:
  flutter_test:
    sdk: flutter
  build_runner: ^2.1.5

{
  "dynamic_key": {
    "key1": "value1",
    "key2": "value2",
    "key3": "value3"
  }
}

import 'package:json_annotation/json_annotation.dart';

part 'my_model.g.dart';

@JsonSerializable()
class MyModel {
  Map<String, dynamic> dynamicKey;

  MyModel(this.dynamicKey);

  factory MyModel.fromJson(Map<String, dynamic> json) =>
      _$MyModelFromJson(json);

  Map<String, dynamic> toJson() => _$MyModelToJson(this);
}

flutter pub run build_runner build

import 'dart:convert';

MyModel parseJson(String jsonString) {
  final jsonData = json.decode(jsonString);
  return MyModel.fromJson(jsonData);
}