Implement the Cesium ion geocoder

[azrogers]

This PR implements the Cesium ion geocoder in Unity. The geocoder can be used like so:

 void Start()
 {
     var geocoder = new CesiumIonGeocoder();
     StartCoroutine(GeocodeCoroutine(geocoder));
 }

 IEnumerator GeocodeCoroutine(CesiumIonGeocoder geocoder)
 {
     var task = geocoder.Geocode(
          CesiumIonServer.defaultServer, 
          "<token here>", 
          CesiumIonGeocoderProviderType.Google, 
          CesiumIonGeocoderRequestType.Search, 
          "<search query here>");
     yield return new WaitForTask(task);
     Debug.Log(task.Result.features);
 }

This is in a draft PR for the moment as it is not fully working yet. Some things also left to work out:

• error handling when creating a connection
• returning bounding boxes
• using default token when no token is provided to the method