[google_sign_in] Add server auth code retrieval to gis_client

[T-P-F]

This adds the ability for a Flutter web app to request a server auth code via gis.

Reference docs:
https://developers.google.com/identity/oauth2/web/guides/use-code-model https://developers.google.com/identity/oauth2/web/reference/js-reference#google.accounts.oauth2.initCodeClient

flutter/flutter#62474

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.

[ditman]

(Please put this in a branch other than main on your repo; I've messed up PRs by force-pushing into other's main branches!)

[T-P-F]

@ditman ok, done, new PR

[ditman]

Thanks, skipping this one!