feat: add log to journald feature

The systemd-journald is used in most major Linux distros that use systemd.
This enables logging using the systemd-journald native protocol, instead
of just writing to stderr.

nix/compile.nix

@@ -74,6 +74,7 @@ let
       "metrics"
       "telemetry-otlp"
       "syslog"
+      "journald"
     ]));
 
   featuresStr = lib.concatStringsSep "," rootFeatures;