Support for `$oidc_pkce_enable` directive through OIDC Policy

[shaun-nx]

As a user of NGINX Ingress Controller, I would like to ensure my OIDC Policies can enable PCKE (Proof of key exchange) to further enhance the security posture of my applications

Final part for #1782

UACs:

• Update OIDC policy to enable toggling the $oidc_pkce_enable directive
• Update documentation to detail the implications of this setting

References:

This directive is currently configured in our templates here:
https://github.com/nginxinc/kubernetes-ingress/blob/release-3.7/internal/configs/version2/nginx-plus.virtualserver.tmpl#L92

[github-actions]

Hi @shaun-nx thanks for reporting!

Be sure to check out the docs and the Contributing Guidelines while you wait for a human to take a look at this 🙂

Cheers!