usdzconvert is not compatible with USD-23

[MagicalTux]

Access to the Primvars API has changed between USD-22.03 and USD-23.05 (current version), making usdzconvert fail to run.

This change enables usdzconvert to work with USD-23.05, but tests are needed before this can be merged.

[MagicalTux]

When running arkit checker:

Checking layer <out.usdz>.
Checking package <out.usdz>.
Checking prim </out>.
Checking prim </out/Materials>.
Checking prim </out/Materials/material_0>.
Checking prim </out/Materials/material_0/surfaceShader>.
Checking shader </out/Materials/material_0/surfaceShader>.
Checking prim </out/Materials/material_0/uvReader_st>.
Checking shader </out/Materials/material_0/uvReader_st>.
Checking texture <0/texgen_0.jpg>.
Checking prim </out/Materials/material_0/diffuseColor_texture>.
Checking shader </out/Materials/material_0/diffuseColor_texture>.
Checking prim </out/Geom>.
Checking prim </out/Geom/node_0>.
['MaterialBindingAPIAppliedChecker']

More info:

[MaterialBindingAPIAppliedChecker]:
 A prim providing a material binding, must have MaterialBindingAPI applied on the prim.