Export Blender Point Clouds as `UsdGeomPoints` primitives.
Summary
- Adds the `USDPointsWriter` class
- Adds tests to ensure that animated positions, velocities, radii, and
regular attributes are all written correctly (in a sparse format)
- Adds a new `export_points` operator property, mirroring the existing
`import_points` option
Pull Request: https://projects.blender.org/blender/blender/pulls/126389
460aa3c231