So far this is unused, but it's used in #139165, and that pointed out
some obvious flaws in the code, like the resulting AttributeStorage
being completely ignored, and old CustomData layers not being freed.
Restructure the API a bit to make it simpler to use, and also make it
keep the non-generic layers in CustomData. That's not necessary for
point clouds but it will be necessary for curves (the next step) since
they have vertex groups.
Pull Request: https://projects.blender.org/blender/blender/pulls/139609
8e2c6bc617