support for clustering

[njsoftdev]

Hello!
This is base functionaltiy to support clustering placemarks in Android and iOS.
This PR only add new functionality and add new example

Example how to use:

YandexMap(onMapCreated: (YandexMapController yandexMapController) async {
                  controller = yandexMapController;
                  await controller!.move(point: Point(latitude: 54.790246, longitude: 32.048847), zoom: 10.0);
                  // Clusterized collection created here
                  clusterizedPlacemarkCollection = await controller!.addClusterizedPlacemarkCollection(
                      iconName: '', // 'lib/assets/user.png',
                      textAlign: 'center',
                      textColor: {'r': 211, 'g': 165, 'b': 103},
                      backgroundColor: {'r': 19, 'g': 43, 'b': 32},
                      strokeColor: {'r': 173, 'g': 171, 'b': 118},
		      fontSize: 15
                  );
                  <dynamic>[
                    [54.790246, 32.048847],
                    [54.789960, 32.048933],
                    [54.789998, 32.050607],
                    [54.790072, 32.053096],
                    [54.788770, 32.054276],
                    [54.786203, 32.054426],
                    [54.791448, 32.047774],
                    [54.793147, 32.050821],
                  ].forEach((element) {
                      await controller!.addClusterizedPlacemark(clusterizedPlacemarkCollection!, Placemark(
                                        point: point,
                                       style: const PlacemarkStyle(
                                       iconName: 'lib/assets/place.png',
                                        opacity: 0.9,
                           ),
                       ));
                  });
                },
              )

[cream-cheeze]

There is my uncompleted PR which also adds clustering support and much more:

• adding of nested collections (not only clusterized)
• clearing and removing collections
• adding composite icons
• support for dynamic cluster icons (if you want do display number of items in cluster, for example).

I had no time to make some changes to the PR, so it hang up for a while... I'm afraid that it is almost impossible to friend our branches. Let me finish my PR and then participate in review - I'll try to finish this weekend or next week.

[DCrow]

Closing since clusterization has been implemented in #157