nixos/timesyncd: add extraConfig option

[cdepillabout]

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.

To test this, in my local configuration.nix I added the following:

{
  services.timesyncd.extraConfig = ''
    PollIntervalMaxSec=180
  '';
}

After running nixos-rebuild switch, here is the content of the /etc/systemd/timesyncd.conf file:

[Time]
NTP=0.nixos.pool.ntp.org 1.nixos.pool.ntp.org 2.nixos.pool.ntp.org 3.nixos.pool.ntp.org
PollIntervalMaxSec=180

Here's the output of the two relevant timedatectl commands:

$ timedatectl timesync-status
       Server: 198.13.37.28 (0.nixos.pool.ntp.org)
Poll interval: 4min 16s (min: 32s; max 3min)
         Leap: normal
      Version: 4
...
$ timedatectl show-timesync 
SystemNTPServers=0.nixos.pool.ntp.org 1.nixos.pool.ntp.org 2.nixos.pool.ntp.org 3.nixos.pool.ntp.org
FallbackNTPServers=0.nixos.pool.ntp.org 1.nixos.pool.ntp.org 2.nixos.pool.ntp.org 3.nixos.pool.ntp.org
ServerName=0.nixos.pool.ntp.org
ServerAddress=198.13.37.28
RootDistanceMaxUSec=5s
PollIntervalMinUSec=32s
PollIntervalMaxUSec=3min
PollIntervalUSec=4min 16s
...

You can see that the max poll interval has been updated.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

Pinging some of the people I recognize in the git history for the timesyncd module: @flokli @andir