Support `nix copy` and `nix-copy-closure`

[axelkar]

This would enable using the --target-host and --build-host options of nixos-rebuild with nix-monitored

[ners]

Thanks for the suggestion. What would be the correct behaviour of these subcommands? Bypassing nom entirely?

Can you tell me some example usages and what you would expect to see?

[axelkar]

nix copy installables... --to ssh-ng://machine --log-format internal-json |& nom --json seems to work pretty well already but it hides the SSH key password prompt.

[axelkar]

maralorn/nix-output-monitor#282