Allow for relationship type on exposed variables for BehaviorScripts

[christianfsstudio]

Relationship could be a useful feature to expose for Behavior Scripts, even more for easy to use randomizer scripts that could make assets randomize with respect to another Rel prim.

This feature would require:

• behavior_utils.py:L30:
  • Add a conditional to call CreateRelationship instead of CreateAttribute if the type is Rel. Additionally, no default should be set.
• exposed_variables_widget.py:L74:
  • Set the "default" metadata ONLY if prop_type is not Rel

With just those two changes, a cool new feature could be added.