swh-cron: manifest-based scheduler for recurring tasks

We need a cron-like service to periodically reschedule (= submit relevant celery tasks) recurrent tasks.

Use cases:

ghlister daily updates
ghlister complete re-listing (monthly?)
git cloning/loading of new github repos
git fetching/loading of already known github repos
...

As an architecture, we want the service to read a list of "manifests", that map cron-like periods to swh.core.scheduling.Task objects + their configuration parameters.

As a technological building block we might want to use croniter.

Open design questions:

cron-like (i.e., will //not// catch up with past tasks, e.g., if the daemon was down), or anacron-like (with catchup)

Migrated from T52 (view on Phabricator)

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