This commit moves Curves and Grease Pencil to use `AttributeStorage`
instead of `CustomData`, except for vertex groups. This PR mostly
involves extending the changes from the above commit for point clouds
to generalize to other geometry types.
This is mostly straightforward, though a couple non-trivial places of
note are the joining of Grease Pencil objects (`merge_attributes`), the
"default render fallback" UV for curves objects which was previously
unused at the UI level and just ended up being the first attribute, and
the `update_curve_types()` call in the curves versioning function.
Similar to:
- fa03c53d4a
- f74e304b00
Part of #122398.
Pull Request: https://projects.blender.org/blender/blender/pulls/140936
