Add cron operator

[IyeOnline]

Adds a cron operator that does scheduled execution analogous to the every operator.

• Internally, both operators now use a shared base
• cron expression parsing and handling is done using an external library https://github.com/mariusbancila/croncpp

Fixes https://github.com/tenzir/issues/issues/1684

[jachris]

Besides one minor functional issue, this works great. Thank you! 🚀

And sorry for the flood of comments. Most of them are nitpicks or about things that you'll just get used to very soon when it comes to this code base.

[jachris]

Thank you for the changes. This works as expected now and can be merged once the remaining open conversations are addressed 👍