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