Adding missing `guest` method to `Authentificator` interface

Within `auth` filter provided with `laravel/laravel` the `guest` method of `Authentificator` interface is used. For now the `Guard` class implementing that interface has that method, but interface itself doesn't.
Fix auth contract.
Merge pull request #5939 from aik099/add-missing-intetrface-method-fix

Adding missing `guest` method to `Authentificator` interface