feat: extract some config into modules

modules/nixos/gnupg.nix

@@ -0,0 +1,20 @@
+{
+  lib,
+  config,
+  ...
+}:
+{
+  options = {
+    gnupg = {
+      enable = lib.mkEnableOption "GnuPG agent with SSH support";
+    };
+  };
+
+  config = lib.mkIf config.gnupg.enable {
+    programs.gnupg.agent = {
+      enable = true;
+      enableSSHSupport = true;
+      enableExtraSocket = true;
+    };
+  };
+}