Viewport Vertex Colors on USDPreviewSurface Using DisplayColors Attribute

**Is your feature request related to a problem? Please describe.**
As a user I would like to be able to view vertex colors assigned using Apply Color in the viewport when a USDPreviewSurface material is applied.

**Describe the solution you'd like**
I would like vertex colors to be displayed in an identical manner as if they were applied to a lambert.

Start by reproducing the issue in the viewport with a USDPreviewSurface material and vertex colors assigned through Apply Color. Compare the result with the same geometry using a Lambert material and trace how the DisplayColors attribute is handled. Done means USDPreviewSurface displays vertex colors identically to Lambert.

Written by the indexing model from the issue text.