Archiver Director/Worker: Add copy to backend worker implementation

Actions storage:
- Open cache_content_get to retrieve contents in cache
- sql/upgrades/080: Add stored procedure to read contents from cache

Actions archiver:
- d/control: Add dependency to archiver on python3-swh.objstorage.cloud
- Renamed Archiver(Director|Worker) to
ArchiverWithRetentionPolicy(Director|Worker)
- Add ArchiverToBackend(Director|Worker)
- Add new celery task dedicated for new workers
- Update docstring details

Related T555

requirements.txt

@@ -11,6 +11,7 @@ flask
 # Internal dependencies
 swh.core >= 0.0.23
 swh.objstorage >= 0.0.2
+swh.objstorage.cloud >= 0.0.9
 
 click
 swh.scheduler

[Phabricator Migration user]

Some references in the commit message have been migrated:

• T555 is now swh/meta#555