[google_sign_in] Fix iOS sever auth codes

[stuartmorgan-g]

Fixes an issue where serverAuthorizationTokensForScopes would usually return the results of getRefreshedAuthorizationTokens directly, even though getRefreshedAuthorizationTokens will never include a server auth code. Now, the server auth code is cached when it is returned from authn steps, so that it can be returned when it is requested, since those are different APIs in this plugin (unlike the underlying SDK on iOS).

Fixes flutter/flutter#175313

Pre-Review Checklist

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[gemini-code-assist]

Code Review

This pull request fixes an issue on iOS where serverAuthorizationTokensForScopes would incorrectly return null. The fix introduces caching for the server authorization code, ensuring it's available for later calls. The implementation is sound, correctly handling user changes by associating the cached code with a user ID. The accompanying tests are thorough, covering the new caching logic and various edge cases. I've added one suggestion to improve the readability of a new test case.