feat: Add support for Scheduled Tasks monitoring

[cleptric]

This adds support for our new Cron Monitors feature, to monitor Laravel's Scheduled Tasks.

<?php

namespace App\Console;

use Illuminate\Console\Scheduling\Schedule;
use Illuminate\Foundation\Console\Kernel as ConsoleKernel;

class Kernel extends ConsoleKernel
{
    protected function schedule(Schedule $schedule)
    {
        $schedule->command('emails:send')
            ->everyHour()
            ->sentryMonitor(env('MONITOR_SLUG'));
    }
}

Needs getsentry/sentry-php#1467 to be merged and released first.

Closes #628