Add system-wide cron job analytic rule

[Fabien Bloume (nusantara-self)]

This PR adds an analytic for system-wide cron job changes detection, which is not covered by default analytics in JAMF Protect.

[Thijs Xhaflaire (txhaflaire)]

Hi Fabien Bloume (@nusantara-self)

Thanks for the contribution to this repository, we are going to review the PR as soon as possible.
Feel free in meantime to add any screenshots from the captured alerts during your testing here as well.

Cheers,
Thijs

[Fabien Bloume (nusantara-self)]

Hello Thijs Xhaflaire (@txhaflaire),

Thanks a lot! Here's an example screenshot showing what the alert looks like. Without this custom rule, a direct change to /etc/crontab would go unnoticed / would not trigger an alert.

Cheers,
Fabien