backup: Refactoring of the backup/restore task to support remote storages

The backup task is tightly coupled with usage of the local file system. There is a plan to support not only a local file system, but also a remote object storages to store and retrieve backup files. This change modifies the code structure for the existing backup tasks that allows to re-use gitaly related operations. It doesn't change the flow of the backup task as it is not yet defined what would be needed to support other kind of storages.

Part of: #3633 (closed)