add code challenge/PKCE

[indy-independence]

https://docs.authlib.org/en/latest/client/frameworks.html#oauth-2-0-code-challenge

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.74%. Comparing base (2c0e17a) to head (92c5dc0).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #380      +/-   ##
===========================================
+ Coverage    63.71%   63.74%   +0.02%     
===========================================
  Files           77       77              
  Lines         8401     8401              
===========================================
+ Hits          5353     5355       +2     
+ Misses        3048     3046       -2     

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