Wire up user location API to iOS SDK

[tobrun]

Add iOS integration for user location:

• https://docs.mapbox.com/ios/api/maps/4.9.0/Enums/MGLUserTrackingMode.html
• https://docs.mapbox.com/ios/api/maps/4.9.0/Classes/MGLUserLocation.html
• https://docs.mapbox.com/ios/api/maps/4.9.0/Classes/MGLUserLocationAnnotationView.html

Flutter API is exposed as a flag to show the user location and an enum to define how the location impacts the camera (listed below).

/// The camera mode, which determines how the map camera will track the rendered location.
enum MyLocationTrackingMode {
  None,
  Tracking,
  TrackingCompass,
  TrackingGPS,
}

[tobrun]

done in #76