Make semantics available from Omniverse

[david2611]

When switching to the Omniverse simulator, class and instance segmentation sensors have been removed.

likely steps for fixing:

1. Enabling topic in the carter.usd
2. Update/understand new semantic/instance encoding (label ids to classes) - Note may need to add content to benchbot_robot_controller to use benchbot_msgs/SegmentImgs or update system that interprets ROS message (robot controller callback)
3. Update connections in carter yaml
4. Reupload robots_sim_omni zip and update add-on
5. Check ROS level topic is available and data makes sense