impl(auth): create auth errors based on gax errors

[alvarowolfx]

Improve how auth errors are classified as transient based on gax errors. Previously, we were just checking if the gax::error had an CredentialError as source and if it was transient. But other gax errors can also be considered transient, based on status code, internal reqwest::Error and etc. This PR creates a helper to be reused across the auth crate.

[alvarowolfx]

Split from work on #5310

[codecov]

Codecov Report

❌ Patch coverage is 94.28571% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.79%. Comparing base (f173ee7) to head (a250c51).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auth/src/credentials/mds.rs	0.00%	1 Missing ⚠️
src/auth/src/errors.rs	95.83%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5350   +/-   ##
=======================================
  Coverage   97.79%   97.79%           
=======================================
  Files         222      222           
  Lines       46386    46405   +19     
=======================================
+ Hits        45364    45383   +19     
  Misses       1022     1022           

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[suzmue]

nit: I see in the the tests that the is_transient result we expected changed from this improvement. Might be nice to expand on what exactly has been improved in the description and why we saw that change.

[alvarowolfx]

nit: I see in the the tests that the is_transient result we expected changed from this improvement. Might be nice to expand on what exactly has been improved in the description and why we saw that change.

@suzmue I've updated the PR description to mention why is an improvement and addressed other review comments