Configurable Auth model

[oldskool73]

This simple change makes it possible to use your existing Auth model configured in config/admin.php, so e.g. I'm using :

/*
     * Authentication config
     */
    'auth'                    => [
        'model' => 'App\Models\User',
        'rules' => [
            'email' => 'required',
            'password' => 'required',
        ]
    ],

...which allows my normal users to log in to the admin area. The Sleeping-Owl/AdminAuth stuff is then unused.

Not tested extensively but doesn't seem to be causing any issues here.