Reimplement PKCE to serialize verifier in session

[michael-doubez]

Persist code verifier in session information in order to allow HA Jenkins.

Fixes #290 regression: PKCE information was stored in google's AuthorizationCode flow and not serialized into session.
Current google implementation doesn't allow the get/set of verifier code code so, in our case, PKCE can no longer be used from within google flow. Therefore, this change implements PKCE in the plugin.

Testing done

Unit test now verifies PKCE validity instead of presence only.

Change manually tested with Google, with PKCE activated.

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

[michael-doubez]

@Vlatombe can you please have a look ? Do I need to increment the serialVersionUID ?

[codecov]

Codecov Report

Attention: Patch coverage is 88.37209% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 73.60%. Comparing base (c3d9244) to head (e8fcd0f).
Report is 1 commits behind head on master.

Files	Patch %	Lines
...ain/java/org/jenkinsci/plugins/oic/OicSession.java	86.84%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #291      +/-   ##
============================================
+ Coverage     73.16%   73.60%   +0.44%     
- Complexity      189      194       +5     
============================================
  Files             9        9              
  Lines           775      807      +32     
  Branches        113      115       +2     
============================================
+ Hits            567      594      +27     
- Misses          151      156       +5     
  Partials         57       57              

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[michael-doubez]

Self-review: LGTM

[Vlatombe]

Caused by #288.

Do I need to increment the serialVersionUID ?

Not needed.