Add reference USD prim - options: “make instanceable”

It would be nice if there was an option “make instanceable” in the **Add reference USD prim** menu dialog (accessed through the Outliner's context menu. This could then be set to the default behavior if desired.

**Describe alternatives you've considered**
This can be set for each referenced prim in the AE, but that's time-consuming.

Start at the Outliner's context menu and the Add reference USD prim menu dialog, then compare how instanceability is currently set for referenced prims in the AE. Done means the dialog offers a “make instanceable” option and supports making that choice the default behavior; the issue does not name a test or file to run.

Written by the indexing model from the issue text.