Add OIDC logout endpoint

[shawnhankim]

As a Product Manager,

I want to integrate with IdP's logout endpoint to fully end session between NGINX Plus and OP as the current NJS implementation clear the token itself, and the IdP's authenticated session still exists at the IdP.

AC:

• Add a map variable of $oidc_logout_endpoint as same as authz and token endpoints here (openid_connect_configuration.conf).

• Add a map variable of $post_logout_return_uri to determine where to send browser after RP requests /logout to the OP, and after the RP (_logout) is called by OP.

• Enhance /logout location:

  • Generate query parameters using $oidc_logout_query_params for the $oidc_logout_endpoint.
  • NGINX Plus: clear tokens.
  • Redirect to the $oidc_logout_endpoint to start ending session in the IdP.

• Enhance /_logout location:

  • Redirected by IdP when IdP successfully finished the session.
  • NGINX Plus: Either show a built-in logout page or redirect to the $post_logout_return_uri.