Add a feature ID getter for picking

[ptrgags]

This PR adds two getters:

• Cesium3DTileFeature.featureId (before this, it was only accessible through the private pickedFeature._batchId)
• ModelFeature.featureId (formally the private pickedFeature._featureId)

@lilleyse two questions on this one:

1. Right now I marked these as experimental in case we have better ideas of how to expose feature ID values to the public API. Do we want this experimental flag or is this a good addition to the public API?
2. I also realized it would be easy to expose featuresLength as well to get the total count of features in the Cesium3DTileBatchTable/ModelFeatureTable. Do we want this too?

Updated photogrammetry classification sandcastle

[cesium-concierge]

Thanks for the pull request @ptrgags!

• ✔️ 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.

[lilleyse]

1. I also realized it would be easy to expose featuresLength as well to get the total count of features in the Cesium3DTileBatchTable/ModelFeatureTable. Do we want this too?

My only hestitation is that featuresLength would not be well defined for feature IDs without an associated property table.

(That is, once we actually get picking working in that case. CC #9884 (comment))

I'm good with merging this PR before we make a decision on featuresLength.