chore(auth): Refactor federateToIdentityPool to a usecase

[mattcreaser]

• PR title and description conform to Pull Request guidelines.

Issue #, if available:

Description of changes:

• Moves callback logic from RealAWSCognitoAuthPlugin to coroutine-based usecase.
• Adds code coverage, previously this logic had none.

How did you test these changes?

• Tests pass

Documentation update required?

• No
• Yes (Please include a PR link for the documentation update)

General Checklist

• Added Unit Tests
• Added Integration Tests
• Security oriented best practices and standards are followed (e.g. using input sanitization, principle of least privilege, etc)
• Ensure commit message has the appropriate scope (e.g fix(storage): message, feat(auth): message, chore(all): message)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

❌ Patch coverage is 72.05882% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 55.40%. Comparing base (a798441) to head (bf949fd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3240      +/-   ##
==========================================
+ Coverage   55.24%   55.40%   +0.15%     
==========================================
  Files        1076     1077       +1     
  Lines       31903    31903              
  Branches     4753     4755       +2     
==========================================
+ Hits        17626    17676      +50     
+ Misses      12450    12388      -62     
- Partials     1827     1839      +12     

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