[google_maps_flutter_ios] Improve marker performance

[Colman]

This changes the iOS maps package to improve the map performance when there are many markers being displayed. Specifically, it does the following:

• Hides accessibility elements on the map. This is default behaviour for the Google Maps iOS SDK and it drastically improves performance.
• Uses an icon cache for the marker images. This prevents the icons from being created from options for every marker on the map. They only need to be created for each unique icon.
• Check if a marker property is different from the current one before setting it

https://github.com/flutter/flutter/issues/60750

Pre-launch Checklist

• [ ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ ] I read the Tree Hygiene page, which explains my responsibilities.
• [ ] I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• [ ] I signed the CLA.
• [ ] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [ ] I linked to at least one issue that this PR fixes in the description above.
• [ ] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• [ ] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or this PR is exempt from CHANGELOG changes.
• [ ] I updated/added relevant documentation (doc comments with ///).
• [ ] I added new tests to check the change I am making, or this PR is test-exempt.
• [ ] All existing and new tests are passing.

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