Add ModelExperimentalType enum

[ptrgags]

Added an enum to label models so we know if it's a single glTF or if it's part of a 3D Tileset (and furthermore if it's a .b3dm, .pnts, .gltf, etc.). This will be useful both in #9978 and #9968

@sanjeetsuhag could you review?

[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.

[ptrgags]

@sanjeetsuhag updated! I decided to make ModelExperimentalType private to minimize the API surface area.

[sanjeetsuhag]

Changes look good! Thanks @ptrgags!