add first version of ntp module

modules/ntp/default.nix

@@ -0,0 +1,19 @@
+{ lib, pkgs, config, ...}:
+let
+  inherit (lib) mkOption types;
+in {
+  options = {
+    system.service.ntp = mkOption {
+      type = types.functionTo types.package;
+    };
+  };
+  config = {
+    system.service.ntp = pkgs.callPackage ./service.nix {};
+    users.ntp = {
+      uid = 52; gid= 52; gecos = "Unprivileged NTP user";
+      dir = "/run/ntp";
+      shell = "/bin/false";
+    };
+    # groups.system.usernames = ["ntp"];
+  };
+}