[google_maps_flutter] Add support for custom cluster icon

### Use case

So Clustering in Google map is just landed but we don't have any support for custom cluster icon. There are some package on pub.dev but they are not proper solution to the problem.

### Proposal

Need some properties that can override default cluster marker.
Like this
https://developers.google.com/maps/documentation/android-sdk/utility/marker-clustering

The ClusterManager constructor creates a DefaultClusterRenderer and a NonHierarchicalDistanceBasedAlgorithm. You can change the ClusterRenderer and the Algorithm using the setAlgorithm(Algorithm algorithm) and setRenderer(ClusterRenderer view) methods of ClusterManager.

You can implement ClusterRenderer to customize the rendering of the clusters. DefaultClusterRenderer provides a good base to start from. By subclassing DefaultClusterRenderer, you can override the defaults.