cli.add_forge_now: Open `schedule-first-visits` with sensible defaults

This should ease scheduling the first visits for add-forge-now request.

The following would be enough to fetch and schedule the forge just listed (be it in production or staging):

swh scheduler add-forge-now \
  schedule-first-visits \
    --visit-type git \
    (--visit-type svn \ # if a lister lists multiple kinds of visit, we can mention it )
    --lister-name gitea \
    --lister-instance-name git.afpy.org \
    ( --production | --staging )  # to list only enabled | disabled origins

Future diffs will add other subcommand to that new module.

Related to https://gitlab.softwareheritage.org/infra/sysadm-environment/-/issues/4674

Test Plan

Common function extracted and tests added

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Migrated from D8936 (view on Phabricator)