Add support for GeoJSON foreign members

[tukez]

Could this library support GeoJSON foreign members? https://datatracker.ietf.org/doc/html/rfc7946#section-6.1

In short, Feature, FeatureCollection and Geometry should allow custom fields of any type in addition to the fields defined in the spec. This would allow this library to work with GeoJSON extensions.

As implementation, a field: foreignMembers: Map<String, JsonElement>, would be enough.