backup: support of the Cloud storages

After adding a new sinks for the backups in 643d6a08 (StorageServiceSink to support AWS, Azure, Google storage for the backups, 2021-07-07) we now can integrate them into the backup task. The URL for the files to store is the path provided as a flag to the binary. It should include a specific schema prefix if a service storage needs to be used (it also requires additional env vars to be set). The provided path will be used as a bucket on the storage service where all backups will be placed.

Closes: #3633 (closed)

Changelog: added