Support GET requests on /application/o/token

[archont94]

Is your feature request related to a problem? Please describe. I want to use authentik as oauth2 for docker registry (distribution/distribution on GitHub). Unfortunately it uses GET requests for /application/o/token endpoint instead of POST (see https://github.com/distribution/distribution/blob/main/docs/content/spec/auth/token.md for details, TLDR: To respond to this challenge, the client will need to make a GET request to the URL).

Describe the solution you'd like It would be great if authentik could support both POST and GET requests on this endpoint.

Additional context I'm not sure why they decided to use GET and if there is any RFC which covers which version should be used, but I assume that adding support for both in authentik wouldn't be problematic.

This is the request docker client does to authentik, in order to obtain token: "GET /application/o/token?account=USERNAME&client_id=docker&offline_token=true&service=SERVICE_VALUE_FROM_REGISTRY_CONFIG HTTP/1.1" 400 140 "-" "docker/24.0.7 go/go1.20.10 git-commit/311b9ff kernel/5.15.153.1-microsoft-standard-WSL2 os/linux arch/amd64 UpstreamClient(Docker-Client/24.0.7 \x5C(linux\x5C))". Client ID seems to be hardcoded, but it can be set to any value in authentik so that should work. SERVICE_VALUE_FROM_REGISTRY_CONFIG is value from config.yml registry file, section auth.token.service.