[google_maps_flutter_platform_interface] Add onPointOfInterestTap support

[tenninebt]

This is the platform interface portion of #11872

Adds PointOfInterestId, PointOfInterestTapEvent, and GoogleMapsFlutterPlatform.onPointOfInterestTap. The default implementation returns an empty stream so existing platform implementations are not broken.

Fixes flutter/flutter#60695 (platform interface only; app-facing and implementation packages land in follow-up PRs per the federated plugin contribution process).

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• I added new tests to check the change I am making.
• All existing and new tests are passing.

@stuartmorgan-g — this is the first sub-PR from #11872 as requested.

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[gemini-code-assist]

Code Review

This pull request adds support for tapping points of interest (POIs) on the map by introducing PointOfInterestId, PointOfInterestTapEvent, and the onPointOfInterestTap stream. Feedback suggests enhancing the API by creating a PointOfInterest class that includes the POI's name and coordinates, rather than just its ID. Additionally, it is recommended to throw an UnimplementedError in the default implementation of onPointOfInterestTap to maintain consistency with other platform interface methods, and to update the corresponding tests accordingly.

[gemini-code-assist]

The native Google Maps SDKs on both Android (OnPoiClickListener) and iOS (mapView:didTapPOIWithPlaceID:name:location:) provide the name and location (LatLng) of the tapped point of interest, in addition to the place ID. \n\nBy only exposing PointOfInterestId (which only wraps the place ID string), developers will not be able to know the name or coordinates of the tapped POI without performing a separate, paid Google Places API lookup. \n\nTo make this API fully functional and useful, please consider defining a PointOfInterest class containing id, name, and latLng, and have PointOfInterestTapEvent wrap that instead of just the ID.

[gemini-code-assist]

To maintain consistency with all other stream methods in this class (such as onClusterTap, onGroundOverlayTap, onMarkerTap, etc.) and to adhere to the class's documented design principle (which states that default implementations of new methods should throw at runtime to signal unimplemented status), this method should throw an UnimplementedError instead of returning an empty stream.

Suggested change

	Stream<PointOfInterestTapEvent> onPointOfInterestTap({required int mapId}) {
	return const Stream<PointOfInterestTapEvent>.empty();
	}
	Stream<PointOfInterestTapEvent> onPointOfInterestTap({required int mapId}) {
	throw UnimplementedError('onPointOfInterestTap() has not been implemented.');
	}

[gemini-code-assist]

Update this test to expect an UnimplementedError to match the corrected default implementation of onPointOfInterestTap.

Suggested change

	test('onPointOfInterestTap() returns empty stream', () async {
	final Stream<PointOfInterestTapEvent> stream = BuildViewGoogleMapsFlutterPlatform()
	.onPointOfInterestTap(mapId: 0);
	expect(await stream.isEmpty, isTrue);
	});
	test('onPointOfInterestTap() throws UnimplementedError', () {
	expect(
	() => BuildViewGoogleMapsFlutterPlatform().onPointOfInterestTap(mapId: 0),
	throwsUnimplementedError,
	);
	});