Add USD Export option to exclude geometry meshes

**Is your feature request related to a problem? Please describe.**
Currently, geo meshes are always exported. It would be nice to have an option to exclude them from export.
Two potential use cases for this are:
1) exporting the materials and bindings, without the meshes. This would be useful when exporting the material portion of a component asset to be layered over the geometry portion.
2) exporting the animated point cache without the meshes. This would likewise be useful to manage the size of animation caches.

**Describe the solution you'd like**
Add the option to exclude geometry/ meshes from the USD export. This should however still include material bindings and animated point data.

**Describe alternatives you've considered**
Currently, we code this with the API. This involves exporting everything and then stripping out the mesh data. Having an option to not write mesh data in the first place would of course be more efficient.

**Additional context**
Add any other context or screenshots about the feature request here.

Start by locating the USD export entry point and the existing geometry-mesh writing path. Check how material bindings and animated point data are handled, then verify that an exclusion option omits meshes while retaining both requested data types. The issue provides no file or test names, so the relevant export tests will need to be found.

Written by the indexing model from the issue text.