Add an optional type field on OriginVisitStatus object

The optional nature of the type will allow to avoid migrating the (db) data model right now then we can have this type field in kafka messages in the origin-visit-status topic

Related to swh-scheduler#2443 (closed) Related to #2963 (closed)

---

Migrated from D4848 (view on Phabricator)

swh/model/hypothesis_strategies.py

@@ -174,6 +174,7 @@ def origin_visit_statuses_d():
         dict,
         visit=integers(1, 1000),
         origin=urls(),
+        type=optional(sampled_from(["git", "svn", "pypi", "debian"])),
         status=sampled_from(["created", "ongoing", "full", "partial"]),
         date=aware_datetimes(),
         snapshot=optional(sha1_git()),