Add support for LexRef as a Schema type of inputs

[MarshalX]

the need for implementation happened because of unusual lexicon added here bluesky-social/atproto#2636

useful code snippet for the future:

            if model_type is ModelType.DATA:
                lines.append(f'{INPUT_MODEL} = {model_path}')
            elif model_type is ModelType.RESPONSE:
                lines.append(f'{OUTPUT_MODEL} = {model_path}')
            else:
                raise ValueError('Wrong type or not implemented')