Lister schema: add is_fork and forked_from_url fields

This expands the schema to add fork (boolean yes/no/unknown) and fork source (URL field) if available from the listers, which will be usable for scheduling heuristics (e.g. for GitHub).

assigned to @olasd

Jenkins job DSCH/gitlab-builds #240 succeeded .
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mentioned in merge request swh-lister!531 (merged)

approved this merge request

merged