Add `nix store import` and `nix store export` commands to work with nar/narinfo

[fricklerhandwerk]

This is part of the CLI stabilisation effort.

Description

The commands should convert between store objects and nar/narinfo .
Since we're not settled on a format, require passing a format flag. This allows to add more formats later and set a default once we're confident this something that can be supported in the long term.

Warning

The current export format does not support CA and is not extensible.

Example

$ nix store export <store path> --format tar
# ... tar stream with nar and narinfo files

$ nix store import <path>...
/nix/store/<hash>-<name>
# more paths

TODO: Clarify CLI guideline in terms of reading from stdin

Context

• nix-store --export produces files that can't be processed by any nix nar commands #8727
• Stabilise nix nar and subcommands #8875

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[nixos-discourse]

This issue has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/2023-09-25-nix-team-meeting-minutes-89/33489/1