MAXAR_content_geojson tilesets should be more like vector tiles

GeoJSON data in tilesets using the MAXAR_content_geojson are faithfully translated into glTF geometry and structured metadata, but they don't retain the vector qualites of the original data. The new extensions developed for vector tiles should be used with these tilesets:

* [3DTILES_content_gltf_vector](https://github.com/CesiumGS/3d-tiles/pull/838) indicates that the tile content is vector content;
* [KHR_mesh_primitive_restart](https://github.com/KhronosGroup/glTF/pull/2569) to preserve line strings in an efficient way;
* [EXT_mesh_polygon](https://github.com/KhronosGroup/glTF/pull/2570) to preserve topology of polygons, currently lost in the translation to triangles.

Start by tracing how MAXAR_content_geojson tilesets translate GeoJSON into glTF geometry and structured metadata. Review 3DTILES_content_gltf_vector, KHR_mesh_primitive_restart, and EXT_mesh_polygon; done means vector content, line strings, and polygon topology are preserved through the translation.

Written by the indexing model from the issue text.