[6.x] Adds Validated auth event

[DarkGhostHunter]

This PR adds the Validated auth event. This event is fired after a User is retrieved from the User Provider and properly validated, but before the Login event and logic:

This allows any package to intercede on the authentication after a successful attempt but before login logic. As you can see, the Login event is fired after all login logic is already executed.

This allows to intercede into the authentication without having to retrieve the user two times. For example, we can now run complex logic and deny further authentication after the user is validated using a Listener:

public function handle(Validated $event)
{
    if ($event->user->isDynamicallyLocked()) {
        response()->view('auth.locked', ['user' => $event->user])->throwResponse();
    }
}

This is handy, for example, when we need to filter what Users can continue to authenticate depending on a complex prerequisites (like a relation value, or an external API response, whatever) which we cannot set in the LoginController, and without having to edit the SessionGuard or macro it.

Please comment so I don't feel alone. Thanks.

[DarkGhostHunter]

I'll comment myself. Good job.

[driesvints]

@DarkGhostHunter thanks man 👍