Add config option to enable PKCE

[michael-doubez]

Add pkceEnabled configuration option to enable PKCE challenge. Note that google library doesn't allow to configure the verification method: it uses S256 if possible and then fallback to plain.

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov]

Codecov Report

Merging #191 (526a646) into master (8bebdeb) will increase coverage by 0.24%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #191      +/-   ##
============================================
+ Coverage     72.94%   73.18%   +0.24%     
- Complexity      174      177       +3     
============================================
  Files             9        9              
  Lines           669      675       +6     
  Branches        106      107       +1     
============================================
+ Hits            488      494       +6     
  Misses          130      130              
  Partials         51       51              

Impacted Files	Coverage Δ
...va/org/jenkinsci/plugins/oic/OicSecurityRealm.java	69.71% <100.00%> (+0.35%)	⬆️

📣 We’re building smart automated test selection to slash your CI/CD build times. Learn more