Add support for logout from OAuth provider

[philippjfr]

Until now we never logged the user out of the OAuth provider we simply cleared the cookies. This is "usually" the desirable behavior since often the OAuth provider is part of a larger SSO system and you don't want to log someone out of Google/GitHub/Gitlab/Azure/Okta when you're simply wanting to log out of the app. Here we make it possible to log the user out of Okta and make the logout endpoint configurable for the GenericLoginHandler.

• Add tests
• Update docs

[codecov]

Codecov Report

Attention: Patch coverage is 34.14634% with 27 lines in your changes missing coverage. Please review.

Project coverage is 81.51%. Comparing base (241b45e) to head (f990144).
Report is 332 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/auth.py	34.14%	27 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6935      +/-   ##
==========================================
- Coverage   81.57%   81.51%   -0.06%     
==========================================
  Files         323      323              
  Lines       47509    47550      +41     
==========================================
+ Hits        38754    38762       +8     
- Misses       8755     8788      +33     

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