Unified Backups: Introduce `.7z` archive format with default compression

Proposal

The Unified Backup currently relies on the old archive code based on compression and decompression commands.

We are changing the approach from using a customizable compression and decompression command, which defaults to gzip, to rely on p7zip custom archive format: .7z. See more here: https://7-zip.org/7z.html

The new archive format supports a variety of compression algorithms, really large files and strong encryption.

We intend to initially ship this using defaults, but allow for compression profiles later on via: #428514 (moved)

• Add documentation for the compression format