Migrate to new storage.origin_visit_add endpoint
This makes use of the latest origin_visit_add endpoint (which takes an iterable of OriginVisit)
The visit_date can no longer be a string so this also deals with it. The cli still allows to input a visit_date as string though.
Depends on swh-storage!875 (closed) (storage)
Test Plan
tox
Migrated from D3264 (view on Phabricator)