Allow inherrited BuiltValueHooks

[Leptopoda]

When making use of interfaces one might wish to add hooks to it that should be invoked by the subclasses.
An example is when setting default (initializeHook) and validating the builder (finalizeHook).
Currently one has to manually add the builders to the subclasses.
Is this a feature you are interested in?

@BuiltValue(instantiable: false)
abstract interface class TestInterface {
  String get value;
  @BuiltValueHook(initializeBuilder: true)
  static void _defaults(TestInterfaceBuilder b) {
    b.value = '123';
  }

  @BuiltValueHook(finalizeBuilder: true)
  static void _validate(TestInterfaceBuilder b) {
    if (b.value!.isEmpty) {
      throw Error();
    }
  }
}

abstract class Test implements TestInterface, Built<Test, TestBuilder> {
  factory Test([void Function(TestBuilder)? b]) = _$SuperObject;
  const Test._();

  static Serializer<Test> get serializer => _$superObjectSerializer;

  @BuiltValueHook(initializeBuilder: true)
  static void _defaults(TestBuilder b) {
    TestInterface._defaults(b);
  }

  @BuiltValueHook(finalizeBuilder: true)
  static void _validate(TestBuilder b) {
    TestInterface._validate(b);
  }
}

[davidmorgan]

Thanks for filing! Sorry for the slow response, I've been on vacation.

That does seem like it makes sense, unfortunately I suspect it's only landable as a breaking change or via extra configuring (inheritBuilders: true), neither of which is ideal.

I'll put it in the pile of possible changes for a closer look at some point, but I don't have a lot of bandwidth for built_value improvements so I suspect it will be a while before I get to it.