Author animated shader attributes in a way that they can render in hydra

[sebastienblor]

Hydra materials don't support animated attributes. We receive a list of VtValues for each attribute and we can't sample them through the shutter.
Because of that, animated shaders in arnold (which today can only happen with matrix_interpolate) only return a single value.

To fix this, when we author usd files with animated shader attributes, we want to author them as an array of multiple values for every time sample. This way we'll get all the keys in an array, instead of a single one

[autodesk-oss-arnold-bot]

Issue synced internally to ARNOLD-15407