seg backup/mirror command

source: https://discord.com/channels/687972960811745322/886550095749873666/1452020965570515107

essentially, create some kind of backup of datadir. Most of the data is in immutable snapshots, so we should be able to use hardlinks for cheap backups.

- devs/devops sometimes creates backups of files before attempting some commands, specially on new servers.
- useful for dev workflows - create backup, execute some offline commands on it to get it to a desired `datadir state` from which some bug can be reproduced, then create multiple backups of this backup to be able to test things on a particular state.
- users are not always aware of the different offline commands/recovery strategies, and often shoot themselves on foot - they do something wrong or it's not easy to test commands out ("my datadir works, why mess with it?"). Giving them a way to do backups can provide a safer env to try out things, debug things more independently etc.