Add user info endpoint

[shawnhankim]

As a Product Manager of NGINX Management Suite - API Connectivity Management (NMS-ACM),

I want a SPA to show signed-in user information in detail using the /userinfo endpoint of the OP.

As a Product Manager of NGINX Plus,

I want to only disclose the specific claims after NGINX Plus extract all of claims that are received by the OP.

AC:

• Add an endpoint of /userinfo.
• Add a map variable of $oidc_userinfo_endpoint
• Add a map variable of $oidc_userinfo_required_claims
• Receive the OP's response and extract claims using $oidc_userinfo_required_claims.