Add support for Gaussian splats

[lilleyse]

Add support for the new Gaussian splat glTF extensions in KhronosGroup/glTF#2490 that were recently implemented in CesiumJS.

• KHR_gaussian_splatting
• KHR_spz_gaussian_splats_compression

(CesiumJS demo)

Some initial thoughts:

• What changes go in Cesium Native, if any?
• Evaluate Gaussian splat rendering in 3DGRUT / Kit 107.3
• Is there a Fabric representation for Gaussian splats?
• Keep an eye on Apple's Gaussian Splats API schema: APPLE: Add Gaussian Splats API schema PixarAnimationStudios/OpenUSD#3716

This came up on the forum: https://community.cesium.com/t/support-for-khr-gaussian-splatting/42201/2