Add support for marker clustering

**Is your feature request related to a problem? Please describe.**
I'm always frustrated when there are multiple pins in the same location and I can't use marker clustering :)

**Does this feature exist in the Google Maps SDK for android and iOS? Please link the documentation for this feature.**
iOS: https://developers.google.com/maps/documentation/ios-sdk/utility/marker-clustering
Android: https://developers.google.com/maps/documentation/android-sdk/utility/marker-clustering (video inside)
JS: https://developers.google.com/maps/documentation/javascript/marker-clustering

**Describe the solution you'd like**
Support native marker clustering for iOS and Android. I'm currently stuck with the mediocre performance of the JS implementation.

I'd love to help with the implementation, but I don't have much experience writing plugins or even native code, I'm a web developer. Can y'all start by checking how easy or difficult this seems?