Support indexed uniform and vertex primvars in hydra

[sebastienblor]

Currently, uniform primvars that have indices are not translated properly to arnold in hydra (it works fine in usd mode). We have special cases for the hydra primvars translation, but we should make it generic so that all uniform & vertex primvars can be flattened when they have indices

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-17410