Define Queue behaviour for each notification channel

[JamesHemery]

Users of my application can receive notifications via mail, in-app or both, depending on their settings.

All my notifications implement ShouldQueue, as I am sending Mails via API. However, this does affect my in-app notification which use the database channel.

These notifications should be available immediately, but they only become visible after the queue handler has processed the job for the notification.

Is there any way to define the queue behaviour on a channel level? I found this discussion, where this was not adressed: #14785

Initially created by @lpeterke (laravel/ideas#1234)

[JamesHemery]

I think of two different syntaxes:

1. Add a viaConnections method:

    public function viaConnections(): array
    {
        return [
            database' => 'sync',
        ];
    }

2. Add a nonQueued method:

    public function nonQueued(): array
    {
        return ['database'];
    }

The advantage of the viaConnections method is that it solves both this problem and gives granular control over which connections to use.

[JamesHemery]

I've made viaConnections implementation in #45264