Move getUserByProviderUid() method into the Auth contract

[jeromegamez]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.38%. Comparing base (a6781fe) to head (847d02b).
⚠️ Report is 2 commits behind head on 8.x.

Additional details and impacted files

@@             Coverage Diff              @@
##                8.x    #1048      +/-   ##
============================================
+ Coverage     88.36%   88.38%   +0.01%     
  Complexity     1473     1473              
============================================
  Files           145      145              
  Lines          4144     4131      -13     
============================================
- Hits           3662     3651      -11     
+ Misses          482      480       -2     

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