Please allow indexing of all primvars, including the normals

While testing a large set of models, we found the size of the usd file can be significantly reduced by indexing all of the primvars. The maya exporter seems to do this for most primvars. The one we added lastly but ended up giving us big gains where the normals.

You would need to manually create the normals primvar as the system one does not allow indexing. However, this would still be a perfectly well defined usd:

_If 'normals' and 'primvars:normals' are both specified, the latter has precedence.
https://graphics.pixar.com/usd/docs/api/class_usd_geom_point_based.html#ac5427209fe38570a75036a3cb7ebb553_

I have attached a zip with a grid where the normals are indexed and one where this grid was imported into maya and written out, the size almost doubles.

Please add an option in the exporter ( or perhaps always use this?) to index the normals as well.

Thanks,
Koen

[grid.zip](https://github.com/Autodesk/maya-usd/files/4731332/grid.zip)

Start by tracing the Maya exporter path that indexes primvars, using the attached grid.zip examples to compare indexed normals with the current output. Done means the exporter can produce indexed normals through an option or established exporter behavior, while preserving the USD precedence described in the issue.

Written by the indexing model from the issue text.