[google_maps_flutter] Fix custom marker icons on iOS

[stuartmorgan-g]

In #10863 I intermediated asset loading through an asset loader, so that individual controllers weren't using the plugin registrar directly. In doing that, I just updated the types of the ivars, but—incorrectly—not their memory models.

Registrar references are generally weak to avoid possible cycles (the registrar may internally be the same thing in the engine that owns the plugin instance), so the result was a weak reference to the new wrapper object. However, unlike the registrar, which has an externally enforced lifetime longer than the plugin, the wrapper was just an object created locally during map controller initialization, so was invalidated almost as soon as it was passed in. This fixes the controllers to keep strong references to the wrapper (which doesn't own anything, and has a weak registrar pointer, so can't create any cycles).

Fixes flutter/flutter#182149

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[gemini-code-assist]

Code Review

This pull request addresses a regression in custom marker icon handling on iOS. The change modifies the memory management of the assetProvider property in FLTGroundOverlaysController and FLTMarkersController from weak to strong. This ensures the asset provider is not deallocated prematurely. The PR also introduces new tests to verify the correct retention of the asset provider and updates the package version and changelog.