Adds MAXAR_content_geojson

[erikdahlstrom]

Readable spec link: here

[javagl]

I think that this is largely obsolete, with the specification of MAXAR_content_geojson and others now being published at https://github.com/Maxar-Public/3d-tiles/tree/wff1.7.0/extensions/MAXAR_content_geojson

[javagl]

These extensions are now maintained in a fork of the 3D Tiles repo that reflects the development of the 'WFF' (Well-Formed Format).

Links to these extensions are now contained in the main extensions README.md file. The latest version of the MAXAR_content_geojson extension (at the time of writing this) is https://github.com/Maxar-Public/3d-tiles/tree/wff1.7.0/extensions/MAXAR_content_geojson