Add OIDC landing page for NGINX to redirect after successful OIDC login

[shawnhankim]

AC:

• Add a map variable of $oidc_landing_page to determine where to send browser after successful OIDC login from the IdP.

Out of Scope:

• Add a /login location to start OIDC flow as this feature will be part of application level instead of a reference implementation for a simple proxy in this repo regarding the following requirement.

  As a Product Manager,
  
  I want a SPA to start a landing page of Single Page App(SPA) without OpenID Connect (OIDC) flow. In the landing page, I want a SPA to just show default data by calling public APIs without API authorization. 
  
  Afterwards, I want to start the OIDC flow when I just click `login` button to display information in detail by calling private APIs based on API authorization using an access token.