[NixOS] Move ssh functionality into its own file

nixos/ssh.nix

@@ -0,0 +1,20 @@
+{ config, pkgs, ... }:
+{
+  services.avahi = {
+    enable = true;
+    nssmdns = true;
+    publish = {
+      enable = true;
+      domain = true;
+      workstation = true;
+      userServices = true;
+      addresses = true;
+      hinfo = true;
+    };
+    extraServiceFiles = {
+      ssh = "''${pkgs.avahi}/etc/avahi/services/ssh.service";
+    };
+  };
+
+  services.openssh.enable = true;
+}