Retrieve fork information for github repositories in swh.lister.github

Our current database for the github lister doesn't store the "parent" repository of a fork. This prevents us from using the smart updater to its fullest, basing our original clone on the data of the parent.

This can be done by making a new query of the github api for each fork repository.

Current estimate: 15 million fork repos / (5000 queries / hour) / (24 hours / day) = 125 days to populate a new table with a single github account for api access.

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Migrated from T313 (view on Phabricator)