[FEATURE] Meta command schedule with cron-like syntax

[pilosus]

Is your feature request related to a problem? Please describe.
A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]
Relying on external scheduling (e.g. Slack's /remind) is not always convenient, especially for power users.
Since dienstplan users are often have software development background, it would be nice to have a simple cron-like syntax to schedule existing commands, or at least some of them:

• rotate
• who
• (possibly) assign

Describe the solution you'd like
I'd like to have a schedule command:

• @dienstplan schedule 0 10 * * 1 rotate my-rota - rotate rota my-rota every Monday at 10:00 UTC
• @dienstplan schedule 0 11 * * 1-5 who my-rota - show who is duty today in rota my-rota from Monday to Friday at 11:00 UTC

Describe alternatives you've considered
Slack's built-in /remind and Workflow builder. They are fine. But using schedule command may be more natural for software developers who tend to use dienstplan