This moves `PointCloud` to use the recently added `AttributeStorage`
at runtime. Mainly this involves implementing the higher level attribute
API on top, and implementing the RNA API as well. The attribute RNA type
is now backed by either CustomDataLayer or bke::Attribute. For now the
new code is specific to point clouds but next steps can reuse it for
Grease Pencil layer attributes, curves, and eventually meshes.
Point cloud attributes no longer have a name length limit.
Internally, the `AttributeStorage` API is extended with a few additions:
- The data structs have static constructors for convenience.
- A few functions give index-based access to attributes
- A "rename" function is added.
The `Attribute` RNA type now exposes a `storage_type` property.
For now the "single value" option is still unused at runtime, and
accessing the single value data isn't implemented yet.
Pull Request: https://projects.blender.org/blender/blender/pulls/139165
fa03c53d4a