feat(auth): support non-default User account credentials

[harkamaljot]

Implementation Details

• Introduced a Builder struct within user_account.rs to accept authorized_user json from the user.
• Allow user to override the optional scopes, token_uri, and quota_project_id parameters.

Refactoring

• Renamed user.rs to user_account.rs as reading documentation the latter name seemed more coherent and maintain consistency with service_account.rs.

Testing

• Updated unit tests to verify that:
  • Provided scopes, token_uri, and quota_project_id correctly override any defaults.
  • Credentials function correctly with the overridden values.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.83%. Comparing base (20eaea6) to head (27e453e).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1747      +/-   ##
==========================================
+ Coverage   95.79%   95.83%   +0.04%     
==========================================
  Files          50       50              
  Lines        1904     1923      +19     
==========================================
+ Hits         1824     1843      +19     
  Misses         80       80              

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