Add equality joint constaints

[andrewkaufman]

Is this a new feature, an improvement, or a change to existing functionality?

New Feature

Feature Description

equality/joint proivides essential "mimic" behaviour that needs to be reflected in USD via an MjcPhysics schema.

Additional context or alternatives

No response

Code of Conduct

• I agree to follow mujoco-usd-converter Code of Conduct
• I have searched the open issues and have found no duplicates