Old API:
```
site.attach(child_spec)
frame.attach(child_spec)
```
New API:
```
parent_spec.attach(child_spec, frame=frame_name_or_object)
parent_spec.attach(site_spec, site=site_name_or_object)
```
This enables accessing the parent object during attach, which will be used in a follow-up CL to automatically append the child assets to the parent.
PiperOrigin-RevId: 717908589
Change-Id: I0fb27e99694c954cb8ba8a8c98484ffd53a4d6db
Alessio Quaglino
4bb6aeb19f