The idea is to allow iterating over e.g. all ID usages of a node from a
whole nodetree, using the same generic handling as existing 'whole ID'
`foreach_id` code.
This is necessary in some cases wher a sub-data needs to processed
independently from any 'owner ID', e.g. in some copy/paste handling.
This is a pre-requirement for proper fix of nodes copy/paste (see
e.g. #120103).
Pull Request: https://projects.blender.org/blender/blender/pulls/121018
