Add equality connect & weld constraints

[andrewkaufman]

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

New Feature

Feature Description

mujoco equality/connect and equality/weld elements add aditional contraints to the simulation. Each equality should map to a specific UsdPhysicsJoint.

• equality/connect should become a UsdPhysicsSphericalJoint joint
• equality/weld should become a UsdPhysicsFixedJoint

These extra joints should also author the physics:excludeFromArticulation attribute to indicate they are not part of any kinematic tree.

Additional context or alternatives

Note equality/joint, equality/tendon, and equality/flex are considered out of scope for this task, as the mapping is less clear.

Code of Conduct

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