Add HeightFields

[andrewkaufman]

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

New Feature

Feature Description

mujoco height fields are currently not converted to USD.

As USD has no native height field prim, this may require a new typed schema in MjcPhysics first

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

[andrewkaufman]

Per newton#595, deprioritizing height fields in favor of tri-mesh rough-terrains