Add OIDC userinfo endpoint for User-Agent to obtain claims about End-User

[shawnhankim]

As a Product Manager,
I want a User-Agent to obtain claims about End-User to show signed-in user information in detail using the OIDC userinfo endpoint of the OP.

Description:

• Added a map variable of $oidc_userinfo_endpoint
• Added a config example of obtaining claims about End-User using the oidc_userinfo_endpoint.

Reference:

• https://openid.net/specs/openid-connect-core-1_0.html#UserInfo