Option to draw bounding boxes around point instancer and instances on selection

**Is your feature request related to a problem? Please describe.**
It can be challenging to identify if you have a model, point instancer, or instance selected in the viewport.

**Describe the solution you'd like**
In "Point Instancer" USD Selection mode, our users have requested the option to see a bounding box around a point instancer (encompasses all of the instances) when it is selected.

In "Instance" USD Selection mode, it would also be helpful to have the option to see bounding boxes around each individual instance when it is selected (in addition to the bounding box described above).

It would also be helpful to colour-code the bounding boxes. For example, in the past, we've used something similar to:
- In "Point Instancer" selection mode: Green around a point instancer (all instances)
- In "Instance" selection mode: Blue around a point instancer (all instances) and yellow around the individual instance

Thanks!