Support for exporting animated attributes on the USD Preview Surface Shader

**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

Start by locating the USD Preview Surface shader export path and how it handles static scalar values and texture assignments. Check how keyed scalar attributes are currently treated, then define completion as exporting their animation as time-sampled keys on the resulting UsdPreviewSurface shader.

Written by the indexing model from the issue text.