Add Sandcastle for glTF PBR extensions

[jjhembd]

Description

Adds a Sandcastle to demonstrate the new glTF PBR extensions.

• Implement glTF extension KHR_materials_specular #11970
• Implement glTF extension KHR_materials_anisotropy #11988
• Implement glTF extension KHR_materials_clearcoat #12006

Testing plan

Load the glTF PBR Extensions Sandcastle and verify the different models load and render, and try the two lighting options.

Author checklist

• I have submitted a Contributor License Agreement
• I have added my name to CONTRIBUTORS.md
• I have updated CHANGES.md with a short summary of my change
  - [ ] I have added or updated unit tests to ensure consistent code coverage
  - [ ] I have update the inline documentation, and included code examples where relevant
• I have performed a self-review of my code

[github-actions]

Thank you for the pull request, @jjhembd!

✅ We can confirm we have a CLA on file for you.

[ggetz]

Thanks @jjhembd! Do we still need to maintain the dev sandcastle examples now that we have this example?

[ggetz]

Please update the title and description tags.

[jjhembd]

I updated the description. I think the title is auto-replaced elsewhere. All the other Sandcastles have "Cesium Demo" here.

[ggetz]

This is pretty atypical when compared with how we handle imports in other Sandcastle examples. I know it's a bit more verbose, but for more consistency can we use Cesium.Cartesian3, Cesium.DirectionalLight, etc?

[ggetz]

Could we start with the Clear Coat Wicker model? It matches the thumbnail, and also provides a more "real life" use case as compared with the test models.

[jjhembd]

Do we still need to maintain the dev sandcastle examples now that we have this example?

Good point, I removed the dev sandcastles. This should be ready for another look.

[ggetz]

Thanks @jjhembd!