Ability to disable showing instance children with an optionVar

UFE outliner display was hard coded to follow the usdview standard of showing the children of instanced prims. This works for what is functionally a read only tool but when authoring transformations and other things in Maya it can be confusing for users to see children they are not allowed to manipulate, and also selecting an instanced child that is maybe many levels of instancing deep means you have to walk out and test where the instance actually starts by failing to move prims ( maybe it is easier to tell in 2022 but we only just moved to 2020 in production and I suspect we are a year or two away from adopting 2022 ).

Can we please have an option var that disables the traversal of instanced prims so that a user can be more aware of what is actually going on, but also be able to toggle the option to see all of the instanced prims?

Ideally when this traversal is disabled the outliner would only show up to the first instance, and viewport selection would fail upwards to the highest selectable SceneItem.

I think this is where this traversal is being allowed :
https://github.com/Autodesk/maya-usd/blob/600c31abcd68ef3fe0047b28fb5f33f6edd0872f/lib/mayaUsd/ufe/UsdHierarchy.cpp#L72

Start with lib/mayaUsd/ufe/UsdHierarchy.cpp around line 72, where instanced-prim traversal is allowed. Trace how that traversal affects the UFE outliner and viewport selection, then define the optionVar behavior: disabling it should stop at the first instance and selection should resolve to the highest selectable SceneItem; toggling it should restore all children.

Written by the indexing model from the issue text.