feat(gax): new type to represent credential errors

[coryan]

CredentialError represents problems trying to create the
authentication credentials. Some of these errors are recoverable, e.g.,
trying to fetch access tokens from the metadata server. Other errors are
not, e.g., malformed credential files.

Part of the work for #1542

[codecov]

Codecov Report

Attention: Patch coverage is 95.45455% with 1 line in your changes missing coverage. Please review.

Project coverage is 95.94%. Comparing base (e94de76) to head (76a8547).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gax/src/error/credentials.rs	95.45%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1688      +/-   ##
==========================================
- Coverage   95.94%   95.94%   -0.01%     
==========================================
  Files          47       48       +1     
  Lines        1850     1872      +22     
==========================================
+ Hits         1775     1796      +21     
- Misses         75       76       +1     

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