Add support for joint mimic constraints when API is available

[scpeters]

Desired behavior

Gazebo Harmonic and newer supports joint mimic constraints (see documentation and the harmonic demo world animated below).

harmonic_mimic_mechanisms_color_fixed.mp4

Since Isaac Sim supports mimic constraints using the PhysxSchemaPhysxMimicJointAPI (see an example in the Isaac Sim URDF importer), it should be possible to copy joint mimic constraint configuration from SDFormat to USD in this repository, but that API is not part of OpenUSD and is not yet publicly available, so this cannot be implemented yet.

cc @rgasoto

Alternatives considered

Implementation suggestion

Additional context