Make a standard way of handling 'add/remove subform on value changed'

[piotruela]

There is a quite common case in which we want to add different type of subform based on the selected value in the parent form.

enum Type { url, document, page }

class ItemFormCubit extends FormGroupCubit {
  ItemFormCubit()
    : typeField = SingleSelectFieldCubit<Type?, ValidationError>(
        initialValue: Type.url,
        validator: Validators.requiredFieldValidator,
        options: Type.values,
      ),

      urlItemFormCubit = UrlItemFormCubit(),
      fileItemFormCubit = FileItemFormCubit() {
    registerFields([typeField]);

    typeField.stream.distinct().listen((data) => _onTypeUpdated(data.value));
  }

  final SingleSelectFieldCubit<Type?, ValidationError> typeField;
  final UrlItemFormCubit urlItemFormCubit;
  final FileItemFormCubit fileItemFormCubit;

  Future<void> _onTypeUpdated(Type? type) async {
    if (type == Type.url) {
      addSubform(UrlItemFormCubit());
    } else {
      urlItemFormCubit.resetAll();
      await removeSubform(urlItemFormCubit, close: false);
    }

    if (type == Type.document) {
      addSubform(fileItemFormCubit);
    } else {
      fileItemFormCubit.resetAll();
      await removeSubform(fileItemFormCubit, close: false);
    }
  }
}

We should find a way to handle those cases in more robust way