Add login endpoint

[shawnhankim]

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.

AC:

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