[google_maps_flutter] Clone cached elements in GoogleMap

[ppicas]

Description

Create a clone of cached elements in GoogleMap (Polyline, Polygon, etc.) to detect modifications if these objects are mutated instead of modified by copy.

Related Issues

flutter/flutter#39737

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[sergicastellsague]

@iskakaushik can you please review this? it fixes the regression

[sergicastellsague]

@iskakaushik friendly reminder! This is ready to be merged as soon as you approve it.

[sergicastellsague]

@amirh do you think you can help us out to push this through?

[iskakaushik]

LGTM