[google_maps_flutter] Add heatmap support

[Rexios80]

Adds heatmap support for web, iOS, and Android

List which issues are fixed by this PR. You must list at least one issue.

#33586
#86811

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

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/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin 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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[Rexios80]

Some notes:

• All platforms seem to be in a pretty good spot. Android and iOS might need some extra work, but I think it's ready for a preliminary review to make sure the API is how we want it.
• The CI is complaining about web/index.html not containing the license text. Should this file have it? None of the other generated files seem to have it. Is there a whitelist somewhere?

[Rexios80]

I wrote integration tests for iOS and Android. I'm pretty sure web was already taken care of.

[stuartmorgan]

Do the native SDKs document what the units of the radius are supposed to be?

[stuartmorgan]

I forgot to say the new integration test structure looks good, thanks!

[Rexios80]

Not sure why my comments didn't show up all nice like yours, but I added some

[stuartmorgan]

We've just completed the migration of the plugin code to the flutter/packages repository, as described in https://flutter.dev/go/flutter-plugins-repo-migration, and this repository is now being archived. Unfortunately that means that all in-progress PRs here must be moved to flutter/packages.

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Our apologies that your PR was caught in this one-time transition. We're aware that it's disruptive in the short term, and appreciate your help in getting us to a better long-term state!