Add Routine support

[Zabuzard]

More and more commands need to execute code on a scheduled routine (for example every x minutes or once per day or once at a specific date).

The command system should offer a generic way to all commands (also, not just SlashCommands) to schedule their actions on a central managed scheduler.

The interface should be easy and convenient to use for all registered commands. It should at least offer ways to:

• execute code at date/time (send birthday greetings to zabu at 8th november)
• execute code after a specific waiting time (expire this message in 2 days)
• repeatedly execute code (check the audit logs every 5 hours)
• optionally persist a schedule, so that it is not lost if the bot is shutdown (/remind me at ... command)

The exact needs are not yet known but commands wanting to use such mechanisms are slowly comming (mod audit log, top helper, free command, ...)

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Edit: Removed any sort of persisted-one-shot-tasks from the scope of this issue (see comments for why).

[github-actions]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days.

[Zabuzard]

This issue is stale because it has been open 30 days with no activity. Remove stale label or comment or this will be closed in 5 days.

This is actually still relevant, we need it eventually.