Add geojson external secondary instances

[lognaturel]

https://forum.getodk.org/t/add-support-for-geojson-external-secondary-instance/36114

https://forum.getodk.org/t/ability-to-load-locations-into-the-geowidget-to-guide-data-collection/27406/12

Would make sense to add docs for CSV at the same time: #88