fix(auth): Rename ServiceAccountCredential and MDSCredential

[sai-sunder-s]

This change is a part of an effort to Consistently use the term "Credentials" instead of "Credential".

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.78%. Comparing base (56135ea) to head (122dc90).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1712   +/-   ##
=======================================
  Coverage   95.78%   95.78%           
=======================================
  Files          49       49           
  Lines        1922     1922           
=======================================
  Hits         1841     1841           
  Misses         81       81           

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