[9.x] Add ability to set middleware based on notifiable instance and channel

[AndrewMonty]

Discussed in #44766

I want to apply middleware to my queueable notifications, but only for specific channels and/or users. For example; no middleware for database notifications, but apply a rate limiter to email and sms notifications - unless the notifiable is an admin.

My suggestion is to pass $notifiable and $channel to the middleware() method on queuable notifications, similar to how withDelay works.

Now I can change my middleware stack depending on the notifiable and channel of each job instance.

class ImportantNotification extends Notification implements ShouldQueue
{
    use Queueable;

    public function via($notifiable)
    {
        return [
            'email',
            'database',
            'broadcast',
        ];
    }

    public function middleware($notifiable, $channel)
    {
        if ($notifiable instance of User && $notifiable->isAdmin()) {
            return [];
        }

        if ($channel == 'email') {
            return [new RateLimited('mailgun')];
        }

        return [];
    }

    ...
}

Notification::send($users, new ImportantNotification);