test(auth): add integration tests for mds sourced id token

[alvarowolfx]

Add integration tests for MDS sourced ID Tokens. Also makes google_cloud_auth::credentials::idtoken and google_cloud_auth::credentials::mds::idtoken public.

Towards #3449 #3587

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.78%. Comparing base (5bf695d) to head (2679f33).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3485   +/-   ##
=======================================
  Coverage   95.78%   95.78%           
=======================================
  Files         120      120           
  Lines        4820     4820           
=======================================
  Hits         4617     4617           
  Misses        203      203           

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