This PR adds vertex groups to `CurvesGeometry` as well as an attribute read/write accessor.
This is also in preparation for GPv3. Since the goal is to have full compatibility with the current grease pencil features, vertex groups need to be supported.
Grease Pencil allows filtering by vertex group for modifiers.To support this, it also makes sense to have read/write access for vertex groups in the attributes API.
In the future, vertex groups should be just another custom attribute on meshes/curves/grease pencil. There are some more issues to be solved before that can happen. This step gets us a bit closer since the vertex weight data is stored in `CustomData`.
Pull Request: https://projects.blender.org/blender/blender/pulls/106944
bc7034d9fd