nixos: move ssh to its own module

modules/linux/ssh.nix

@@ -0,0 +1,22 @@
+{ config, lib, pkgs, ... }:
+
+{
+  # Enable the OpenSSH daemon.
+  services.openssh = {
+    enable = true;
+    allowSFTP = true;
+    openFirewall = false;
+    authorizedKeysInHomedir = false;
+    settings = {
+      PermitRootLogin = "no";
+      PasswordAuthentication = false;
+      GatewayPorts = "no";
+    };
+  };
+
+  # enable ssh in the firewall
+  networking.firewall.allowedTCPPorts = [ 22 ];
+
+  # enable fail2ban
+  services.fail2ban.enable = true;
+}