This enables znapzend users to specify its full configuration through
NixOS options, without ever needing to use the stateful znapzendzetup
command.

This works by running znapzendzetup with the specified config in
ExecPre, just before the znapzend daemon is started.

There is also the pure option which will clear all previous znapzend setups,
making it as stateless as can get, as only the setup declared in
configuration.nix will be persisted.

Here is the configuration I'm using for one of my machines:

{
  services.znapzend = {
    enable = true;
    autoCreation = true;
    pure = true;
    zetup."main/data" = rec {
      plan = "15min=>5min,1h=>15min,1d=>1h,1w=>1d,1m=>1w";
      recursive = true;
      destinations.backup = {
        dataset = "betty/backup";
        plan = plan + ",1y=>1m";
        presend = "zpool import -f betty | true";
      };
    };
  };
}

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.