Placemark needs a structured initializer

[1ec5]

Placemark and its subclasses need initializers besides the init(from:) that takes a Decoder. Any initializer – even one that takes no parameters – would enable client code to create arbitrary placemarks for testing or to create adapters with other search sources.

Currently, the only way to create a Placemark or GeocodedPlacemark is to decode one from Carmen GeoJSON. Swift’s type system makes it awkward to manipulate JSON. Turf’s GeoJSON API doesn’t help a whole lot, because Carmen GeoJSON expects properties strewn about at the top level instead of inside the properties object.

/cc @mapbox/navigation-ios @avi-c