Add to "extras" during GLB export.

[OwenSignaturize]

It is currently not possible to add to extras when performing a GLB export as far as I can tell.

I would like for the extras field to be added to GLTFast.Schema.Node (and other relevant areas), and not just to GLTFast.Newtonsoft.Schema.Node.

Alternatively being easily able to extend the GameObjectExporter and/or GLTFWriter to enable custom manipulation of the Json Serialization step of GLB Export would be fine.

Im currently having to modify the source code in my project to allow for this, as I require GLB export. Because this repo isnt up to date with version 6.6 (the version I get when adding the package "com.unity.cloud.gltfast" via package manager, I cant even just fork this repo to get things to work I have to copy files from the PackageCache, and make modifications to it locally.

If I've missed something then it would be great if the documentation could explain more clearly how to achieve adding extras to GLB files.