Expose authenticator as an injectable (service)

[andreespirela]

The main idea is that developers should be able to use Mandarine's built-in authentication as they want. This means, they should have access to the internal class in the core that executes authentication.

We will be exposing this as refactors are done #208

With this, Mandarine's authentication system can be used not only for webapps but also for CLI apps and any other app that is coded in Mandarine as Mandarine grows.