Add `NewtonSceneAPI`

[andrewkaufman]

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

New Feature

Feature Description

Add an API for scene level attributes. Intentionally limiting this to timeStep, maxSolverIterations, and enableGravity for now.

Additional context or alternatives

No response

Code of Conduct

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

[andrewkaufman]

As agreed in the design meeting, we're adding enableGravity as well. Still no solver-specific attributes.