chore: move OIDC PKCE support from UI to backend

[yann-soubeyrand]

Hello,

During my investigation regarding an issue we were facing with SSO login involving PKCE, I dig into the OIDC code of the server. I saw that the PKCE support was not in the OIDC code of the server, but in the UI code. I do not know why it has been done this way, but it seems to me that it is simpler to support PKCE in the code of the server and avoid duplicating part of the code in the UI. That is why I am proposing this change.

The advantages are:

• Removal of a JS dependency.
• Avoiding code duplication.
• Addition of tests.
• Ability to use an OIDC client secret.
• Ability to use an HTTP only secure cookie for the token.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• The title of the PR conforms to the Toolchain Guide
• I've included "Closes [ISSUE #]" or "Fixes [ISSUE #]" in the description to automatically close the associated issue.
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• Does this PR require documentation updates?
• I've updated documentation as required by this PR.
• I have signed off all my commits as required by DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My build is green (troubleshooting builds).
• My new feature complies with the feature status guidelines.
• I have added a brief description of why this PR is necessary and/or what this PR solves.
• Optional. My organization is added to USERS.md.
• Optional. For bug fixes, I've indicated what older releases this fix should be cherry-picked into (this may or may not happen depending on risk/complexity).

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[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 55.61%. Comparing base (b4a63ae) to head (1d605cd).

Files with missing lines	Patch %	Lines
util/settings/settings.go	0.00%	5 Missing ⚠️
util/oidc/oidc.go	85.18%	4 Missing ⚠️
util/test/testutil.go	62.50%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21729      +/-   ##
==========================================
+ Coverage   55.59%   55.61%   +0.01%     
==========================================
  Files         340      340              
  Lines       57419    57441      +22     
==========================================
+ Hits        31924    31943      +19     
+ Misses      22807    22795      -12     
- Partials     2688     2703      +15     

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