[gsi_interface] Introduces canAccessScopes and userDataEvents.

[ditman]

Introduces: canAccessScopes method and userDataEvents stream to the platform_interface of the google_sign_in plugin.

Default implementations:

• canAccessScopes => throw not implemented.
• userDataEvents => null (currently user data events cannot be introduced asynchronously from the native side)

The changes above enable the separation of Authentication and Authorization, and asynchronous sign-in operations as needed (the web needs this now).

Issues

• Starts landing: [google_sign_in] Adds "Button" Sign In for Web. #3461
• Enables fixing: [google_sign_in_web] Display Sign In With Google button flutter#121781
  • Enables fixing: Build dashboard sign in silently is failing flutter#121275

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.

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

[stuartmorgan-g]

LGTM with nit

[ditman]

Triggering a branch update to see if the "Windows" checks get unstuck.

[auto-submit]

auto label is removed for flutter/packages, pr: 3635, due to - The status or check suite Linux plugin_tools_tests has failed. Please fix the issues identified (or deflake) before re-applying this label.

[ditman]

My dearest autosubmit bot, please take a look at this PR :')