Use Clients in ratpack-pac4j

To be able to "gather" all clients within the same callback url.

I kept what was designed initially with the Pac4jModule which provides a global protection for an application, but I changed the underlying handlers to make them interact with Clients. A simple as previously, but more extensible...

Here is the demo to test it: https://github.com/leleuj/ratpack-pac4j-demo.