Handle Apple Sign-In via OAuth2

[stefanorosanelli]

This PR provides changes to OAuth2Authenticator class to support Apple Sign-In process, where:

• a JWT leeway configuration may be needed
• return call from authorization URL is performed via POST
• id_token must be used for user verification

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.24%. Comparing base (6131a1a) to head (e555107).
Report is 2 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #90      +/-   ##
============================================
+ Coverage     92.17%   92.24%   +0.07%     
- Complexity      238      241       +3     
============================================
  Files            26       26              
  Lines           741      748       +7     
============================================
+ Hits            683      690       +7     
  Misses           58       58              

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