Implement the randomStorePath method for the daemon store

Add a new method to the daemon API to allow directly querying a random
path on the remote side.

Also makes sure that the tests will only run when the daemon version is
high-enough

Tagging release 2.9.1

Tagging release 2.9.0

Tagging release 2.8.1

Add a cli command exposing the randomStorePath method

```console
$ nix store random-path
/nix/store/ivl1p0m612n3zj1hm9cw50gxdzxa5mlb-util-linux-minimal-2.37.4-lib
```

Generalize for potentially any store implem

Tagging release 2.8.0

Make builtins.randomStorePath semantically correct

Tagging release 2.7.0

Tagging release 2.6.1