Add MAXAR_mesh_variants extension

[nithinp7]

Add support for the MAXAR_mesh_variants extension:

• Add auto-generated files for the extension.
• Small fixes in the glTF code generator to enable generating the extension.

For an example front-end integration using this extension see: CesiumGS/cesium-unreal#773.

For logistical reasons, includes the changes from #463

[nithinp7]

@kring I finally opened this, it is just the auto-generated code and the improvements to the generator that you made. I had made an earlier attempt at creating a workaround in the generator by introducing the "manuallyDefined" option in the glTF.json. I used the new option in tools/generate-classes/index.json. If that option isn't necessary after your generator changes, feel free to remove it (or let me know and I'll remove it).

[kring]

Thanks @nithinp7. Please update CHANGES.md in main to mention the new extension.