Update UsdUVTexture to single-output signatures in 1.39

[jstone-lucasfilm]

This is a proposal to update the UsdUVTexture node to single-output signatures in MaterialX 1.39, allowing these nodes to robustly distinguish between color3, color4, vector3, and vector4 outputs. Version upgrade logic would be used to update UsdUVTexture references in earlier documents to the new node signatures.

Additional details on the background and motivation for this change can be found in #1372.