feat: Add support for Cron Monitoring

[cleptric]

This adds support to send Cron Monitor check-ins as envelopes to Relay.

<?php
$checkIn = new CheckIn(
    monitorSlug: getenv('MONITOR_SLUG'),
    status: CheckInStatus::inProgress(),
);

$event = Event::createCheckIn();
$event->setCheckIn($checkIn);

$this->hub->captureEvent($event);

// do stuff

$event = Event::createCheckIn();
$event->setCheckIn(new CheckIn(
    id: $checkIn->getId(),
    monitorSlug: getenv('MONITOR_SLUG'),
    status: CheckInStatus::ok(),
));

$this->hub->captureEvent($event);

[antonpirker]

looks good to me (but "me" is no php expert) :-)

[stayallive]

This makes it obvious we need a refactor at some point to stop dealing with non-envelopes, but other than that, no comments 💪