Add options.id to ModelExperimental

[j9liu]

This PR adds the id option to ModelExperimental, allowing an object to be associated with the model when it is picked. Model stores its pick ids in a _pickIds array, but in ME they are also stored in _pipelineResources (which I renamed from _resources). Is it important to avoid the duplication here?

For testing: this impacts the "Clamp to 3D Tiles" sandcastle, but that uses the Entity API. So to check that it works, go to ModelVisualizer and:

1. Change Model.fromGltf to ModelExperimental.fromGltf,
2. Replace the computation in getBoundingSphere: just call BoundingSphere.clone(model.boundingSphere, result), no need to apply the model matrix to it.

[cesium-concierge]

Thanks for the pull request @j9liu!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.

[ptrgags]

Looks good to me, thanks @j9liu!