Allow mayaUSDImport command to import point instancer to maya instancer

**Is your feature request related to a problem? Please describe.**

Currently there seems to be no import translator functionality for a USD Point Instancer.
Additionally, even though you can "Duplicate as USD data" from a maya instancer you cannot convert back with "Edit as Maya data"

**Describe the solution you'd like**

Allow instancer to be converted with its prototypes to maya data, preferably also supporting animated point data.

**Describe alternatives you've considered**

Potentially a custom import chaser for `mayaUSDImport` or register a custom USD translator which would also allow the Edit as maya data functionality.

However, from a Maya perspective I'm still not sure how trivial that'd be since I don't know what nodetypes/datastructures are available to maya to have time samples point data with e.g. attributes, etc. that can be saved/embedded with the maya scene file.

Also, I believe custom USD import translators are C++ only?

**Additional context**

I assume the lack of this is mostly due to Maya's native tools to be lacking in regards to the Instancer (and its editability) of just point data that can also be animated since I believe by default there's no data structure in which one can interactively add/remove points with transforms easily other than particles which aren't meant for manual editing. MASH does provide some tools, but those as far as I know would still disallow trivial storage for animated data local to the scene.

Start by investigating the mayaUSDImport entry point and the existing custom import chaser or USD translator options mentioned in the issue. Determine which Maya data structures can represent USD Point Instancer prototypes and animated point data, then define completion against conversion through Edit as Maya data.

Written by the indexing model from the issue text.