Add 'store add-gc-root' command to manually add indirect roots

[layus]

Motivation

This is a PoC fix for #7138. Now that I implemented it, I do not think it is the most elegant way to do it, as there is no way to ensure that the paths are not deleted during this operation. The command could fail however if not all the paths were successfully rooted.

Context

#7138

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[Mic92]

Intuitively I would have expected such as subcommand here:

Suggested change

	$ nix add-root foo
	$ nix store add-gc-root foo

[layus]

I wanted to add it in nix gc add-root, but nix gc seems mostly unused and intended to start a gc.

[roberth]

We currently don't have a top-level nix gc subcommand group, but a nix store gc command.

To help keep some consistency between commands, the subcommands are ideally nix <noun> <verb> with the exception of a small number of frequently used commands.
gc is a verb, since the "garbage" part is subordinate to the "collect" part (as a noun adjunct to be technical).
Since our commands end in verbs, this would create the expectation that nix gc is not a subcommand group, but a single action.

I think nix store add-root would be ok, but perhaps nix store add-gc-root would be better, as I think the hint about gc would be welcomed when reading scripts that call this command.
Interactive use should not be common, because that would usually come after nix build, which already produces a root.

[layus]

Who makes final decisions about that ?

[Mic92]

Nix team members, I would say. Often happens in those regular meetings.

[layus]

Changed name, added tests.