rig and animation publish needs to handle multiverse compounds

[pberto]

already implemented:

• rigger imports character model
• rigger sets up skeleton, binds skin and creates control
• rig instance creator sets up the three sets for geom, controls and rig
• anim publish will output a sparse usd file with animated P and N

to be implemented:

• rigger loads a usd asset for the background/environment (a compound is created)
• rigger adds compounds inside the rig instance creator
• rigger creates & adds the transform overrides (locators) that the animator is supposed to animate
• rigger adds the locators in the controls set (we could also skip this and detect from the compound which transform overrides exist as they are input connections)
• anim publish will output one usda override for each compound in the set

To be verified and to understand how OP will present these multiple usda for each asset in there loader