Enhance AuthProvider to support oauth2/OIDC flow

[jowilf]

close #218

[github-actions]

🚀 Deployed on https://docs-preview-221--starlette-admin-docs-preview.netlify.app

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (86a6044) 100.00% compared to head (eafcf5c) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #221   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           68        68           
  Lines         4939      4961   +22     
=========================================
+ Hits          4939      4961   +22     

Files Changed	Coverage Δ
starlette_admin/auth.py	100.00% <100.00%> (ø)
starlette_admin/base.py	100.00% <100.00%> (ø)
starlette_admin/contrib/odmantic/admin.py	100.00% <100.00%> (ø)
starlette_admin/contrib/sqla/admin.py	100.00% <100.00%> (ø)
starlette_admin/helpers.py	100.00% <100.00%> (ø)

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[simonsax]

I tried it with my project and was able to implement oauth2 authentication. Thanks a lot!

[sternalon]

Looking forward to this one being merged into the main branch!