Add OIDC end session endpoint and custom query params

[shawnhankim]

Issue:

• https://github.com/nginxinc/nginx-openid-connect/issues/70

Summary:

• Added the IdP's end session endpoint to terminate the user session on the IdP's side.
• Added the customizable variable to support different query parameters from each IdP.
• Enhanced RP's callback URI for NGINX to clean cookies and redirect to the OIDC logout landing page.

Description:

• Added a map variable of $oidc_end_session_endpoint as same as authorization and token endpoints in the openid_connect_configuration.conf.

• Added a map variable of $oidc_logout_landing_page to determine where to redirect browser after successful logout from the IdP.

• Added a map variable of $oidc_end_session_query_params to support different query parameters per each IdP.

• Enhanced /logout location:

  • Add query parameters using $oidc_end_session_query_params for the $oidc_end_session_endpoint.
  • NGINX Plus: cleared tokens.
  • Redirected to the $oidc_end_session_endpoint to start ending session in the IdP.

• Enhanced /_logout location:

  • Redirected by IdP when IdP successfully finished the session.
  • Clean cookies
  • NGINX Plus: Redirect to the $oidc_logout_landing_page.

[shawnhankim]

@route443 :

• Thanks for your review in detail for the PR.
• This PR is to simplify from the previous PR.
• For you to easily manage this repo to reduce any concerns from the enhancements based on the reviews on the PR, I have divided a big PR into small PRs, and this is one of PRs.
• Let me know if you have any questions.