[Feature Request] Add methods to DataBuilders to copy nested fragment data class properties to the current data class

[tpucci]

Hey!

I think it would be awesome to have methods to copy nested fragment data to the current builder.

My use case is to create a lot of mocks that could import one another for easier maintenance.
But without the possibility to use a fragment data inside a query data builder, it makes this a lot more complicated.

Here is an example (notice the fromHumanFrag method):

fragment HumanFrag on Human {
    id
    birthday
    height(unit: METER)
}

query HumanQuery($id: ID!) {

    human(id: $id) {
        ...HumanFrag
    }
}

  test('can serialize and deserialize responses with nested fragment', () {
    final human = GHumanFragData((b) => b
      ..id = '123'
      ..height = 180.0
      ..birthday = DateTime(2021, 1, 1));

    final res = GHumanQueryData((b) => b
      ..human.fromHumanFrag(human));

    final json = res.toJson();

    final resFromJson = GHumanQueryData.fromJson(json);

    expect(res, resFromJson);
  });