fix: add PKCE config key to config schema

[alnr]

Follow-up to #4078

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.50%. Comparing base (5830ffb) to head (246b59d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4098      +/-   ##
==========================================
+ Coverage   78.48%   78.50%   +0.01%     
==========================================
  Files         376      376              
  Lines       26755    26755              
==========================================
+ Hits        20998    21003       +5     
+ Misses       4155     4151       -4     
+ Partials     1602     1601       -1     

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