Adding new optional parameter auth_signin URL

[rafaelfelix]

Thanks a lot for this plugin! Definitely a missing piece on kong's open source ecosystem (after all, the oauth2 plugin won't work in DB-less mode).

The idea of this patch is to check if the HTTP GET to auth_uri returns 401, then the proxy redirects the user to the URL specified in auth_signin. This reproduces the same functionality of https://kubernetes.github.io/ingress-nginx/examples/auth/oauth-external-auth/, thus allowing the use of a single sign-on URL when applicable.

The patch is meant to be backwards-compatible, as the auth_signin parameters is optional.