Bring `Cesium3DTilesTerrainProvider` into `main`

[ggetz]

We have a long-running branch—3d-tiles-terrain— that adds a new Cesium3DTilesTerrainProvider class. This is used for rending global 3D Tiles via the existing terrain engine.

This terrain provider supports only a subset of 3D tilesets (ie. 3D Tiles 1.1 with implicit tiling schemes matching that of terrain), so it may not quite ready for prime time yet. As such we should use the experimental tag to give us some flexibility if things need to change.

In the interest of maintenance as well as making this early work generally available, let's get this branch updated and merged into main. We'll also want to ensure the testing, documentation, and API structure are up to par as a part of this.

[ggetz]

@lilleyse Are there other specific requirements for 3D tilesets using this feature that should be called out in the docs?

[lilleyse]

3D Tiles loaded with Cesium3DTilesTerrainProvider must follow this structure:

• Conforms to the same WGS84 double-headed quadtree tiling scheme as quantized mesh. Must have two root tiles with region bounding volumes and implicit tiling.
• Multiple contents are not allowed
• Tile metadata must include TILE_MINIMUM_HEIGHT, TILE_MAXIMUM_HEIGHT, TILE_BOUNDING_SPHERE, TILE_HORIZON_OCCLUSION_POINT semantics
• Only GLB is supported, not glTF
• Each GLB must have a single node, mesh, and primitive
• Primitive must have POSITION attribute
• Primitive must have NORMAL attribute if requestVertexNormals is true
• Primitive must have indices with UNSIGNED_SHORT or UNSIGNED_INT component type
• Attributes and indices must be tightly packed
• If requestWaterMask is true, primitive must reference an EXT_structural_metadata property texture with WATERMASK semantic.
• Primitive must have CESIUM_tile_edges extension
• Model may be compressed with EXT_meshopt_compression / KHR_mesh_quantization