feat(k8s): declare 'crossplane' module

modules/k8s/crossplane.nix

@@ -0,0 +1,12 @@
+{ config, lib, ... }:
+
+{
+  options.k8s.crossplane.enable = lib.mkOption {
+    default = false;
+    type = lib.types.bool;
+  };
+
+  config = lib.mkIf config.k8s.crossplane.enable {
+    nixpkgs.allowedUnfree = [ "upbound" ];
+  };
+}