nixos/timesyncd: add extraConfig option

This adds an `extraConfig` option to timesyncd for setting additional
options in `/etc/systemd/timesyncd.conf`.

This is similar to things like `services.journald.extraConfig` and
`services.logind.extraConfig`.

nixos/modules/system/boot/timesyncd.nix

@@ -20,6 +20,18 @@ with lib;
           The set of NTP servers from which to synchronise.
         '';
       };
+      extraConfig = mkOption {
+        default = "";
+        type = types.lines;
+        example = ''
+          PollIntervalMaxSec=180
+        '';
+        description = ''
+          Extra config options for systemd-timesyncd. See
+          <link xlink:href="https://www.freedesktop.org/software/systemd/man/timesyncd.conf.html">
+          timesyncd.conf(5)</link> for available options.
+        '';
+      };
     };
   };
 
@@ -35,6 +47,7 @@ with lib;
     environment.etc."systemd/timesyncd.conf".text = ''
       [Time]
       NTP=${concatStringsSep " " config.services.timesyncd.servers}
+      ${config.services.timesyncd.extraConfig}
     '';
 
     users.users.systemd-timesync.uid = config.ids.uids.systemd-timesync;