[google_maps_flutter_ios] Add Advanced marker support

[jokerttu]

This PR adds Advanced markers support to the iOS implementation of google_maps_flutter.
Approved combined PR: #7882
Approved and merged platform interface PR: #9737
Issue: flutter/flutter#155526

Blocker:
GMSAdvancedMarker is not rendered when the marker icon is a GMSPinImage instance. Tracking issue:
https://issuetracker.google.com/issues/370536110

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[stuartmorgan-g]

Picking up the discussion from the main issue here: do you have a link to the tracking issue for the blocker here ("an issue with the GMSPinImage not rendering on the map", per this comment)? As noted in that discussion, the link is to a long-closed issue.

[jokerttu]

Picking up the discussion from the main issue here: do you have a link to the tracking issue for the blocker here ("an issue with the GMSPinImage not rendering on the map", per this comment)? As noted in that discussion, the link is to a long-closed issue.

Added tracking issue information to this PR description:
https://issuetracker.google.com/issues/474141340
Edit: the "long-closed issue" is now re-opened: https://issuetracker.google.com/issues/370536110

[stuartmorgan-g]

Please make this non-nullable to avoid a repeat of flutter/flutter#182540 on iOS (and for consistency with the changes in #11061).