Use cesium-native branch of CesiumGS/glTF as the code gen source.

[kring]

The code generation for the glTF classes was previously pulling its schemas from:
https://github.com/CesiumGS/glTF/tree/3d-tiles-next

With this PR, it's pulling them from:
https://github.com/CesiumGS/glTF/tree/cesium-native

The two branches are currently identical, so this doesn't change much to start. But the idea is that it:

1. Gives us an official source for the glTF spec used by cesium-native,
2. Allows us to incorporate extensions that are not yet ratified or even officially blessed as part of 3D Tiles,
3. Insulates the cesium-native codegen from surprising upstream changes (we take changes when we mean to, not necessarily every time we re-rerun the code generator).

We should do PRs into https://github.com/CesiumGS/glTF/tree/cesium-native when we want to bring in upstream changes or incorporate new extensions.

CC @weegeekps @keyboardspecialist @lilleyse