Use `SharedAssetSystem` for schemas in `MAXAR_content_geojson`

In #1402 we had to do a workaround to pass the schema from `MAXAR_content_geojson` to external tileset loaders, due to incorrect URLs in target test data. Since they're all supposed to reference the same schema object, we should change this to use the `SharedAssetSystem` for the schema in the future.

Start by reviewing MAXAR_content_geojson and the workaround described in #1402, then trace how its schema is passed to external tileset loaders. Compare that flow with SharedAssetSystem usage. The work is done when the schema uses SharedAssetSystem consistently and the workaround for the target test data is no longer needed.

Written by the indexing model from the issue text.