It would be great to be able to have `ExtraMember` concept for a GeoJSON feature. This is also in line with the GeoJSON specification [https://www.rfc-editor.org/rfc/rfc7946#section-5](https://www.rfc-editor.org/rfc/rfc7946#section-5).