Support for exporting animated attributes on the USD Preview Surface Shader

[dgovil]

Is your feature request related to a problem? Please describe. The USD Preview Surface shader currently only supports export of static values or texture assignments. For scalar values, it would be handy to be able to animate these attributes.

In fact , it's even odder right now because it just exports no values when keyed.

Describe the solution you'd like I would like for animated values to be exported as time sampled keys on the resulting UsdPreviewSurface Shader