[google_maps_flutter] Add AdvancedMarker to exports

[yuruyuri16]

Export AdvancedMarker in google_maps_flutter.
Otherwise, users would need to import google_maps_flutter_platform_interface to be able to use it.

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Code Review

This pull request exports the AdvancedMarker class from the google_maps_flutter package by adding it to the export list in google_maps_flutter.dart. This makes the class available to users of the package without a separate import.

Please note that the repository's contribution guidelines require pull request titles to be prefixed with the package name in square brackets (e.g., [google_maps_flutter]). Please update the title accordingly.