Add support for RP-initiated OIDC logout

[route443]

Implement support for RP-initiated logout in accordance with OpenID Connect RP-Initiated Logout 1.0. Introduce the oidc_end_session_endpoint variable to specify the end_session_endpoint URL.

If oidc_end_session_endpoint is not set or is empty, the default behavior of logging out only on the NGINX side is maintained. When set, the endpoint triggers the RP-initiated logout as specified in the OIDC specification.

This PR is based on the revised PR #87 initially submitted by user @llomgui. Thank you to @llomgui for the initial implementation and contribution.

Summary of Changes

• Added oidc_end_session_endpoint variable to specify the OIDC end session endpoint URL.
• Updated the logout function to:
  • Handle RP-initiated logout by redirecting to the specified end_session_endpoint.
  • Include logic to renew ID token if refresh token is available, but session_jwt is expired.
  • Fall back to traditional logout if both tokens are absent.

[llomgui]

Hey @route443,

Do you plan to create a PR to update the current files https://github.com/nginxinc/kubernetes-ingress/tree/main/internal/configs/oidc with your latest changes?
Or do you want me to update my PR nginx/kubernetes-ingress#4986 ?

Thank you with this merge.