[EMSUSD-1528] Support proxy shape Maya instancing

[ppt-adsk]

Maya instancing of proxy shape nodes is unsupported. Instancing proxy shape nodes does not draw the instanced copies in the viewport.

If this is valuable to customers, instancing of proxy shapes should be supported.

USD instancing is different and useful, but does not serve exactly the same purpose.

Code such as MayaUsdProxyShapeBase::parentTransform() does not consider the path, and simply uses the first path. StagesSubject::stageChanged() translates USD notifications to a single UFE path; if proxy shapes are instanced, we would need multiple notifications, one per path.