Added callback URLs for code exchange and logout

[pankajupadhyay29]

Problem:
Could not provide dynamic code exchange URL, custom session start after code exchanges and session end during logout.
Background:

To use NGINX as an API Gateway for various products we need to add context information with a callback URL for the product which needs a dynamic URL for code exchange based on the product settings
Similarly, we need a callback for products

Description:

• made code exchange redirect location a map so that it can be set based on URL
• Added two new variables for After Code Exchange redirect and initial logout URL

[route443]

Hello @pankajupadhyay29,

1. Regarding the after_codexch_redir variable, we already have PR Add OIDC landing page for nginx to redirect after successful OIDC login #74, which introduces a new oidc_landing_page variable:

map $host $oidc_landing_page {
    # Where to send browser after successful login. If empty, redirects User 
    # Agent to $request_uri.
    default "";
    #www.example.com $redirect_base;
}

This name better reflects the meaning of this variable.

2. Regarding the redir_location variable, it appears that your use case is quite specific. While I understand the flexibility you are aiming for, altering the default configuration for all users may not be the most appropriate course of action. + It's worth noting that you already have the capability to customize redir_location from "/_codexch" to any URL you desire. Therefore, I'm not sure if it's necessary to make this change in the default configuration for all users.

3. Regarding the initial_logout_redir variable, your proposed change seems redundant and could introduce unnecessary confusion. The existing oidc_logout_redirect serves a similar purpose, and if initial_logout_redir is defined, oidc_logout_redirect would essentially be ignored. This raises the question: what prevents you from using oidc_logout_redirect in place of initial_logout_redir? Furthermore, after redirecting the user via oidc_logout_redirect, you can easily send them to a custom location where you can implement any additional logic, such as a njs-based handler, if the standard functionality lacking.