[google_maps_flutter_web] Add Marker drag events

[JamesMcIntosh]

Add events to track start of drag and position during drag for Google Maps Flutter Web

Issue: flutter/flutter#26117
Platform changes: #2653
Flutter app: #2838

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]. (Note that 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].
• [/] I updated CHANGELOG.md to add a description of the change.
• [/] I updated/added relevant documentation (doc comments with ///).
• [/] I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• [/] All existing and new tests are passing.

[JamesMcIntosh]

@stuartmorgan looks like the might still be a problem with the publishable check for google maps flutter. This is what the error message is saying when there is both a README.md and CHANGELOG.md file in that package.

�[31mERROR: Package validation found the following errors:
* The pubspec is hidden, probably by .gitignore or pubignore.
* You must have a LICENSE file in the root directory.
  An open-source license helps ensure people can legally use your code.
Package validation found the following potential issues:
�[0m
�[31mERROR: * Please add a README.md file that describes your package.
* Please add a`CHANGELOG.md` to your package. See https://dart.dev/tools/pub/publishing#important-files.
Sorry, your package is missing some requirements and can't be published yet.
For more information, see: https://dart.dev/tools/pub/cmd/pub-lish.
�[0m
�[31mERROR: pub finished with exit code 65
�[0m
Unable to publish google_maps_flutter_web

[stuartmorgan]

See #4388

[ditman]

This branch needs to be updated with master for it to pass the publishable check. @JamesMcIntosh if you give us permissions in the PR (there's a checkbox near the bottom when creating a PR that says something similar to: "Allow edits from maintainers"), I can do some of these chores for you while reviewing the code.

[JamesMcIntosh]

Both branches already had the "allow maintainers" permission checked, just did a rebase and push

[JamesMcIntosh]

@stuartmorgan Now the submit-queue is failing and the "details" link now take you to a page full of JSON rather than the summary.
https://api.github.com/repos/flutter/plugins/check-suites/3911833499/check-runs

This is the format of the other links (not pointing to the api subdomain).
https://github.com/flutter/plugins/pull/4385/checks?check_run_id=3748907644

[stuartmorgan]

@stuartmorgan Now the submit-queue is failing

That's a live check indicating that the tree is currently closed so nothing can land. You don't need to do anything.

and the "details" link now take you to a page full of JSON rather than the summary.

We don't control the format of the check, it's an external app

[ditman]

LGTM! Thanks for including tests, and pushing the updated mocks!