nixos/fail2ban: add extraPackages option

some ban actions need additional packages (eg ipset). since actions can be
provided by the user we need something general that's easy to configure.

we could also enable ipset regardless of the actual configuration of the system
if the iptables firewall is in use (like sshguard does), but that seems very
clumsy and wouldn't easily solve the binary-not-found problems other actions may
also have.

nixos/modules/services/security/fail2ban.nix

@@ -62,6 +62,16 @@ in
         description = "The firewall package used by fail2ban service.";
       };
 
+      extraPackages = mkOption {
+        default = [];
+        type = types.listOf types.package;
+        example = lib.literalExample "[ pkgs.ipset ]";
+        description = ''
+          Extra packages to be made available to the fail2ban service. The example contains
+          the packages needed by the `iptables-ipset-proto6` action.
+        '';
+      };
+
       maxretry = mkOption {
         default = 3;
         type = types.ints.unsigned;
@@ -249,7 +259,7 @@ in
       restartTriggers = [ fail2banConf jailConf pathsConf ];
       reloadIfChanged = true;
 
-      path = [ cfg.package cfg.packageFirewall pkgs.iproute2 ];
+      path = [ cfg.package cfg.packageFirewall pkgs.iproute2 ] ++ cfg.extraPackages;
 
       unitConfig.Documentation = "man:fail2ban(1)";