This enables two workflows:
1) Users that parsed some other format into MjSpec, or built it programmatically can export animated USD easily with as minimal as possible interaction with USD.
2) Creating a spec from USD, then modifying the spec, should not cause a duplication of data when writing to USD.
PiperOrigin-RevId: 827627520
Change-Id: I02c8c14a18569547710e54f3704d52ac0bf70ce6
Sam Haves