This change introduces two new USD API schema:
* MjcEqualityAPI - base API for representing MuJoCo equality constraints in USD, this will typically be applied to joints such as UsdPhysicsFixedJoint.
* MjcEqualityWeldAPI - extends MjcEqualityAPI for MuJoCo's weld equality constraints with an attribute for torqueScale.
The usd_decoder is updated to parse UsdPhysicsFixedJoints and convert the new schema attributes into the appropriate mjsEquality data fields.
PiperOrigin-RevId: 856723634
Change-Id: I0982fd3168b643f8e91d87bb5ae5933e526b63a1