[google_maps_flutter] Add missing exports

[stuartmorgan-g]

This adds exports that should have been in #7882, but I missed in review.

Like much of the google_maps_flutter APIs currently, advanced markers rely on clients using types that are defined in the platform interface package. This re-exports them, and fixes the examples to not import the platform interface.

Ideally we would have more separation between the API layers, but this is deeply ingrained in the API design of the plugin at this point, so changing it would require a comprehensive overhaul.

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[gemini-code-assist]

Code Review

This pull request correctly adds missing re-exports for advanced marker-related classes from the google_maps_flutter_platform_interface package into the main google_maps_flutter package. The example files have been updated to reflect these changes by removing direct imports from the platform interface, which improves the API's consistency and usability. The changelog and pubspec.yaml have also been updated appropriately. The changes are well-aligned with the pull request's objective.

[stuartmorgan-g]

@jokerttu I ran into one potentially thorn issue here that I completely missed in review: one of the example pages is calling GoogleMapsFlutterPlatform.instance.isAdvancedMarkersAvailable and I can't remove that because there's no passthrough from the app-facing package. I was going to add one, but then I noticed that it takes a map ID. But the only way to get a map ID is to have made a MapWidget. And the marker type is a parameter to the MapWidget constructor.

Don't clients need to know whether advanced marker support is available before passing the advanced marker type to MapWidget? Is it a mistake in the platform interface that the query takes a map ID?