[google_sign_in] Add a platform interface method to clear auth tokens

[stuartmorgan-g]

Platform interface portion of #9846

Part of flutter/flutter#173924

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene page, which explains my responsibilities.
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• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to 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 I have commented below to indicate which version change exemption this PR falls under¹.
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• I updated/added any relevant documentation (doc comments with ///).
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Footnotes

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[gemini-code-assist]

Code Review

This pull request adds a new clearAuthorizationToken method to the platform interface, allowing for the clearing of cached access tokens. The changes are well-structured, including the new method in the abstract class, a corresponding parameter class, and a test to ensure the default implementation throws an UnimplementedError as expected. I have one suggestion regarding a minor naming inconsistency in the documentation to improve clarity. Overall, this is a solid addition to the package.