Remove the reliance of SG in material naming.

**Is your feature request related to a problem? Please describe.**
It is very frustrating when trying to make a dcc agnostic pipeline with how reliant mayausd is on the shadergroup convention when converting USD to maya data. Making _SG on the UsdMaterial being enforced as a standard to make sure the usd import from maya or any other dcc comes into maya correctly.

https://github.com/Autodesk/maya-usd/blob/9e3b9c31d4c82d3f12b0d789ea720f5c39655cb5/lib/mayaUsd/fileio/shading/shadingModeExporterContext.cpp#L471

https://github.com/Autodesk/maya-usd/blob/9e3b9c31d4c82d3f12b0d789ea720f5c39655cb5/lib/mayaUsd/fileio/shading/shadingModeExporterContext.cpp#L511

**Describe the solution you'd like**
It was not clear to me if this is just user error, but my ideal solution, instead of having to have heavy pre and post import/export events with mayausd is that this could be toggled off to not make maya a bottleneck on interopbility

**Describe alternatives you've considered**
Alternatives i have discovered... make it mandatory to add SG lol.

**Additional context**
I have tried using a bind script in our assembly layer to just hook the correct materials to the maya specific materials then let maya handle the import that way but, this breaks back and forth with houdini making this pipeline less interoperable without having to do alot of custom work here.

Start with the linked sections of lib/mayaUsd/fileio/shading/shadingModeExporterContext.cpp at lines 471 and 511 to trace where the _SG naming convention is applied. Determine how an option could disable that reliance without breaking Maya, USD, or Houdini round trips; done means the behavior is controllable and the affected import/export paths remain interoperable.

Written by the indexing model from the issue text.