[google_sign_in] Implement clearAuthorizationToken

[stuartmorgan-g]

Platform interface portion of #9846

Part of flutter/flutter#173924

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[stuartmorgan-g]

No changes from the main PR, which already has platform review, so @bparrishMines this just needs a sanity check.

[gemini-code-assist]

Code Review

This pull request introduces the clearAuthorizationToken method across Android, iOS, and web platforms for the google_sign_in plugin. The Android implementation leverages the native clearToken API, the iOS version is a no-op as the SDK handles token invalidation automatically, and the web version clears a local token cache. The changes include necessary dependency updates, pigeon interface modifications, and tests for the new functionality. My feedback includes a suggestion to use a more conventional error code format in the Android implementation for better maintainability.

[gemini-code-assist]

The error code "clearAuthorizationToken failed" is rather long and descriptive for an error code. It's more conventional to use a short, machine-readable string (e.g., in snake_case or kebab-case). This would improve consistency with common error handling patterns and make it easier for clients to programmatically handle this specific error.

For instance, clearCredentialState uses "Clear Failed", which is shorter. A more structured code like clear_authorization_token_failed would be even better for consistency and parsing.

Suggested change

	new FlutterError("clearAuthorizationToken failed", e.getMessage(), null)));
	new FlutterError("clear_authorization_token_failed", e.getMessage(), null)));

[stuartmorgan-g]

If we ever start seeing this in practice, this should be switched over to using structured errors anyway; we don't want people doing programatic handling of error code strings that are already inconsistent within and across platform implementations (which is why the structured error guidance exists now). So I didn't worry about making this match anything.

[mdebbar]

web bits look good to me!

[stuartmorgan-g]

Oops, I forgot the main PR didn't have web approval; thanks for catching that and reviewing here!