modules: add option to specify username

Given that I use it in plenty of places, it makes sense to place it
there for reference in other modules.

modules/default.nix

@@ -1,5 +1,5 @@
 # Common modules
-{ ... }:
+{ lib, ... }:
 
 {
   imports = [
@@ -8,4 +8,13 @@
     ./services
     ./system
   ];
+
+  options.my = with lib; {
+    username = mkOption {
+      type = types.str;
+      default = "ambroisie";
+      example = "alice";
+      description = "my username";
+    };
+  };
 }