[google_maps_flutter] Improved perfomance of clusterization

[OlehSv]

Google Maps implementation for flutter web is invoking rendering after each added marker to cluster. It is extremely non optimal, as result app is freezing for some time in case of significant number of markers (200+, but it is more obvious for 1k+) especially if they are using images instead of default icon.

As fix I am proposing to utilize method addItems of JS implementation instead of addMarker and render only as batch of markers are added.

I have tested new implementation with 3k markers and before it was freezed for 15-30 seconds, now it works without any freeze at all.

Fixes #179456

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[OlehSv]

@mdebbar Could you review please? It is quite safe PR, it has minimal changes to specific feature