Add OIDC PKCE configuration through policy

[javorszky]

Proposed changes

Closes #6713

• Adds a PKCEEnable property to the OIDC Policy Spec
• fix whitespace issues in tests, templates, and snapshots
• adds template test that covers PKCE enabled
• adds a PKCE enabled to a failing virtual server config test
• adds a pytest for PKCE flow

PKCE flow oauth client requirements

In the test we use keycloak, so the info is specific to that, however please consult your oauth provider on how to set up a client that will use PKCE. For keycloak you need a client that is

• using standard flow
• is NOT using client authentication
• direct access is disabled
• public client option is enabled (public client: client application can not keep a secret: mobile apps, frontend clients, etc; the other one is confidential client, they can keep a secret, see https://oauth.net/2/client-types/ for more info)
• and the PKCE challenge method is S256

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

[javorszky]

This is a draft while I do manual testing and write Pytests

done, updated PR text above

[codecov]

Codecov Report

Attention: Patch coverage is 86.95652% with 3 lines in your changes missing coverage. Please review.

Project coverage is 52.50%. Comparing base (b422a76) to head (9cbb395).

Files with missing lines	Patch %	Lines
internal/configs/virtualserver.go	76.92%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7765      +/-   ##
==========================================
+ Coverage   52.47%   52.50%   +0.03%     
==========================================
  Files          90       90              
  Lines       21550    21570      +20     
==========================================
+ Hits        11308    11325      +17     
- Misses       9773     9775       +2     
- Partials      469      470       +1     

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[vepatel]

Also need to update the existing OIDC example and policy key in docs

[pdabelf5]

Closes #1782

[javorszky]

Documentation for keycloak in the main doucmentation repo: nginx/documentation#585