Support cron-backed jobs

[Harsh Shandilya]

I currently use hcctl to monitor sync jobs and have them run by cron from the distro. Using a cron parser it should be possible to implement a scheduler within hcctl. Steps to completion would roughly be:

1. Add a cron expression parser (preferably a library dependency) and wire up the CLI to accept and validate it as input
2. Add support for running a task and sleeping till the next run as decided by the cron expression

[Harsh Shandilya]

Guess I'll pick this one up

[Harsh Shandilya]

I'm increasingly of the opinion that this does not really belong in hcctl and relying on existing cron functionality is the safer bet.