sources/oauth: configurable PKCE mode

[BeryJu]

PKCE was already kinda implemented for the twitter source, this makes it configurable and adds S256 support

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[codecov]

Codecov Report

❌ Patch coverage is 96.42857% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.93%. Comparing base (06bfcf0) to head (b03ba10).
⚠️ Report is 13 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
authentik/sources/oauth/clients/oauth2.py	89.47%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #17487   +/-   ##
=======================================
  Coverage   92.92%   92.93%           
=======================================
  Files         868      868           
  Lines       47833    47906   +73     
=======================================
+ Hits        44448    44520   +72     
- Misses       3385     3386    +1     

Flag	Coverage Δ
e2e	45.19% <44.64%> (-0.03%)	⬇️
integration	23.18% <25.00%> (-0.01%)	⬇️
unit	91.07% <96.42%> (+0.01%)	⬆️
unit-migrate	91.12% <96.42%> (+0.01%)	⬆️

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authentik PR Installation instructions

Instructions for docker-compose

Add the following block to your .env file:

AUTHENTIK_IMAGE=ghcr.io/goauthentik/dev-server
AUTHENTIK_TAG=gh-b03ba102a267881f455545580ef124abf6ec95fd
AUTHENTIK_OUTPOSTS__CONTAINER_IMAGE_BASE=ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s

Afterwards, run the upgrade commands from the latest release notes.

Instructions for Kubernetes

Add the following block to your values.yml file:

authentik:
    outposts:
        container_image_base: ghcr.io/goauthentik/dev-%(type)s:gh-%(build_hash)s
global:
    image:
        repository: ghcr.io/goauthentik/dev-server
        tag: gh-b03ba102a267881f455545580ef124abf6ec95fd

Afterwards, run the upgrade commands from the latest release notes.