Previously, the attribute accessor were defined in the `geometry_component_*.cc`
files. This made sense back in the day, because this attribute API was only used
through `GeometryComponent`. However, nowadays this attribute API is independent
of `GeometryComponent`. E.g. one can use `mesh.attributes()` without ever
creating a component.
The refactor contains the following changes:
* Move attribute accessors to separate files for each geometry type. E.g. from
`geometry_component_mesh.cc` to `mesh_attributes.cc`.
* Move implementations of e.g. `Mesh::attributes()` to `mesh.cc`.
* Provide access to the `AttributeAccessorFunctions` without actually having a
geometry. This will be useful to e.g. implement
`attribute_is_builtin_on_component_type` without dummy components.
Pull Request: https://projects.blender.org/blender/blender/pulls/130516
4da580236b