Parse tar data backed up via stdin

[dionorgua]

Hi,

It would be cool to have a way to 'import' tar archive as stdin backup.

Something like

tar c <....> | rustic backup -

Why? Sometimes it's too hard to get access to actual data that needs to be saved.
For example proxmox have vzdump command that can save backups to .tar.gz/zstd format on disk or can output tar stream to stdout. Surely we can 'unpack' that tar temporary but this extra I/O that can be avoided

Same as restic/restic#2226