port ghlister to swh task interface - list all / catch up

This will allow to schedule one job, every now and then, to //re-list all of github//. The intended use case is either restarting from scratch the listing, or catching up with private repositories that have been made public since last complete listing.

The implementation of this might require multiple sub-tasks, tasked with listing specific sub-parts of the repository id space.

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Migrated from T38 (view on Phabricator)