As described in #122398, implement read and write support for a new
attribute storage system. Currently this is only implemented to support
forward compatibility; the format used at runtime isn't changed at all.
That can be done one step at a time during the 4.5 and 5.0 development
cycles. A new experimental option for testing tells Blender to always
save with the new format.
The main benefit of the new structure is that it matches the attribute
system design, it allows for future attribute storage optimization, and
each attribute is an allocated struct, which will give pointer stability
for the Python API.
The next step is to connect the attribute API and the RNA API to
AttributeStorage for the simplest geometry type, point clouds.
Pull Request: https://projects.blender.org/blender/blender/pulls/133874
