UsdGeomMotionAPI should have an API schema adapter

[robp-sidefx]

At present, the data associated with UsdGeomMotionAPI only ends up on Hydra prims via UsdImagingGprimAdapter which means it can't be used to provide information to, for example, a Hydra Generative Procedural.

I'd like to propose an API schema adapter to serve this data translation purpose instead.

I'll submit a basic PR shortly, but I desperately need expertise & insight into what's missing and whether there are any "blockers" that would suggest it can't/shouldn't be moved out of GprimAdapter.

[jesschimein]

Filed as internal issue #USD-11857

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