Support backing up S3 (and S3-compatible) buckets as a data source

[hplarozner]

Currently Databasement supports S3-compatible storage as a destination for backups, but not as a source. It'd be great to be able to add an S3 bucket as a backup target itself similar to how database servers work now so Databasement could periodically snapshot/sync bucket contents (objects, versions, metadata) to another storage location, on the same schedule/retention system already used for databases.