Making auth_time configurable in OpenID Connect

While trying to implement an OpenID Connect server with this lib, I stumbled upon an issue with auth_time not being configurable. The lib set auth_time to the current time, whereas it should be the time when the End-User authentication occured (according to OpenID Connect specs).

This pull request adds a config parameter "id_auth_time". When present, it'll be used in the ID Token instead of the current time. I'm not sure it's the right way to do it, but I didn't find any other solution to set the auth_time in my application.