nixos.chrony: add extraFlags config option

nixos/modules/services/networking/chrony.nix

@@ -30,6 +30,8 @@ let
     ${cfg.extraConfig}
   '';
 
+  chronyFlags = "-n -m -u chrony -f ${configFile} ${toString cfg.extraFlags}";
+
 in
 
 {
@@ -76,6 +78,13 @@ in
           <literal>chrony.conf</literal>
         '';
       };
+
+      extraFlags = mkOption {
+        default = [];
+        example = [ "-s" ];
+        type = types.listOf types.str;
+        description = "Extra flags passed to the chronyd command.";
+      };
     };
 
   };
@@ -123,7 +132,7 @@ in
           '';
 
         serviceConfig =
-          { ExecStart = "${pkgs.chrony}/bin/chronyd -n -m -u chrony -f ${configFile}";
+          { ExecStart = "${pkgs.chrony}/bin/chronyd ${chronyFlags}";
           };
       };