Implementation of the File, GPX, KML and Geojson providers in 8.x-3.x branch

The attached patch accomplishes this issue task, both on the widgets and formatters sides, for file, gpx, kml and geojson plugin types providers.
Gpx, kml and geojson plugin geocode operations rely upon the geofield.geophp service, and will return a \Geometry (not just POINT but also Linestring, Multilinestring, Polygon, Multipolygon), instead of a \Geocoder\Model\AddressCollection.
That's why some object interfaces and geocode\doGeocode methods definitions have been adapted and un-restricted accordingly (to both return \Geocoder\Model\AddressCollection or \Geometry, or NULL).
All these amends don't interfere with, and don't introduce regressions to, lastly added enhancements to 8.x-3.x-dev branch (such as Convert providers from simple config to a config entity ...),
and have been widely and succesfully tested with real Geocoding use cases of incremental complexity.
Exceptions handlings have been setup and implemented in the code.
This is being set to "Needs review" ... and will be committed into dev if any downside arises from some contributor review.