[google_maps_flutter_web] Initial support for custom overlays

[elitree]

This is a continuation of flutter/flutter#116132 from the now archived plugins repo. Below is the original description by the original submitter @AsturaPhoenix:

Saves tile bytes to blobs and uses img elements to decode and render. Does not implement opacity, perform caching, or serve placeholder images.

Issue: flutter/flutter#98596

Known issues:

• [google_maps_flutter_web] API performance bottleneck in custom tile overlays flutter#116132
• Map marker visual artifacts when magnified AsturaPhoenix/trip_planner_aquamarine#22

I'll open this in draft for now because I need to go through the checklist below.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki 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 listed 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.
• 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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[elitree]

Opened with wrong code identity; will reopen with the correct GitHub one.