[google_maps_flutter_platform_interface] Adds size to BitmapDescriptor (1/2)

[ditman]

Adds a size parameter to the BitmapDescriptor.fromBytes constructor, so web applications can specify the actual physical size of the bitmap.

The parameter is not needed (and ignored) in other platforms. Other platforms seem to be doing actual size / dpi to figure out the correct physical size.

Fixes (part 1) https://github.com/flutter/flutter/issues/73789

Testing

• Added unit tests, both for web and non-web cases
• Deployed repro-app here: https://dit-maps-tests.web.app

Pre-launch Checklist

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

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[ditman]

Thanks for landing this @stuartmorgan, you're a beast! 🙏