Consolidate Cesium3DTilesSelection and Cesium3DTilesReader

[nithinp7]

Our Cesium3DTilesSelection library leverages the (mostly) auto-generated CesiumGltf and CesiumGltfReader libraries to parse gltf and gltf extension properties. This allows us to maintain a lock-step support for specific versions of specs by auto-generating the reader code from the spec directly.

Our 3D Tiles and 3D Tiles extension support on the other hand is handwritten. Cesium3DTilesSelection uses custom, handwritten Tile and Tileset representations as well as handwritten code to parse 3D Tiles and 3D Tiles next tilesets. The Cesium3DTiles and Cesium3DTilesReader libraries are auto-generated and could be used to replace the handwritten code.

Before fully integrating the two auto-generated libraries, we could consider first integrating just the auto-generated parsers. Our ongoing tile loader refactor will make this much easier.