[5.5] allow to specify the queue while scheduling of queued jobs

[pavel-mironchik]

Based on this idea in laravel/internals/issues/457 and inspired by #18235.

Right now you schedule a job to be queued like this:

# /app/Console/Kernel.php

protected function schedule(Schedule $schedule)
{
    $schedule->job(new UpdatePrices)->hourly();
}

My simple addition will allow you to specify the queue:

# /app/Console/Kernel.php

protected function schedule(Schedule $schedule)
{
    $schedule->job(new UpdatePrices, 'update prices')->hourly();
}

You'll need this to "categorize" jobs or prioritize queues. This code change is fully backward compatible with current release.