feat: Cron Monitor upsert

[cleptric]

$checkIn = new CheckIn(
    monitorSlug: 'upsert-monitor',
    status: CheckInStatus::inProgress(),
    release: '1.0.0',
    environment: 'production',
    monitorConfig: (new MonitorConfig(
        schedule: MonitorSchedule::crontab('*/5 * * * *'),
        // or
        schedule: MonitorSchedule::interval(
            value: 30,
            unit: MonitorScheduleUnit::minute(),
        )),
        checkinMargin: 5,
        maxRuntime: 30,
        timezone: 'UTC',
    )),
);

[stayallive]

@cleptric please take another look after my changes, but I think this looks pretty decent!