PKCE not available in OIDC (OpenID Connect) Backend

[mkeuschn]

With the new OAuth 2.1 (https://oauth.net/2.1/) standard PKCE is mandatory

PKCE is required for all OAuth clients using the authorization code flow

For now we can disable PKCE in our Identity Provider but in the future this may not possible.

When PKCE is enabled following error is returned

error_description=code_challenge+is+required

DefectDojo Ticket: DefectDojo/django-DefectDojo#14729

[nijel]

We already have PKCE support in BaseOAuth2PKCE so adding that to OIDC would be easy. The challenge comes with figuring out backend compatibility.

[nijel]

I've quickly created #1680 that wires PKCE into OIDC.