[EMSUSD-188] When exporting USD, Maya place2dtexture nodes are omitted

**Describe the bug**
When exporting USD, Maya place2dtexture nodes are omitted.

**Steps to reproduce**
Apply a texture to a UV mapped object with the usdPreviewSurface shader, transform the texture position with a place2dtexture node, and export the object as USD. Import the USD into Maya again. The place2dtexture nodes are gone, and the transformation of the texture placement is lost.

**Expected behavior**
I expect the exporter to keep the store the texture transforms in the USD file, and the importer to convert them back into place2dtexture nodes.