wip oauth token with keychain storage

[asoucar]

Contribution Summary

When google OAuth2 login flow is complete, uses code to request token. Successful request creates oath object with token, expiration date, and refresh token. Retries up to 3 time if unsuccessful.
WIP: current issues with credential storage, repeated access to credential storage for authentication. Server type assumed imap, needs to be set properly. Deleting credentials previously implemented but not used

AI Contribution Disclosure

• This contribution does not include any changes created or assisted by AI.
• This contribution includes changes assisted by AI.

Contribution Checklist

• I have read and affirm that my contribution adheres to Mozilla’s Community Participation Guidelines
• This contribution adheres to the Thunderbird iOS Contribution Guidelines.
• This contribution does not contain merge commits, and is rebased on the latest from the iOS codebase.
• This contribution includes tests for any new functionality, and maintains tests for any updated functionality.
• This PR has a descriptive title and body that accurately outlines all changes made, and contains a reference to any issues that it fixes (e.g. Closes #XXX or Fixes #XXX).