cluster/lib: implement config.lib.forService for better option filtering

cluster/default.nix

@@ -17,6 +17,7 @@ lib.evalModules {
     ./lib/mesh.nix
     ./lib/secrets.nix
     ./lib/testing.nix
+    ./lib/lib.nix
 
     ./import-services.nix
   ];

cluster/lib/lib.nix

@@ -0,0 +1,12 @@
+{ config, lib, ... }:
+
+{
+  options.lib = {
+    forService = lib.mkOption {
+      description = "Enable these definitions for a particular service only.";
+      type = lib.types.functionTo lib.types.raw;
+      readOnly = true;
+      default = service: lib.mkIf (!config.simulacrum || lib.any (s: s == service) config.testConfig.activeServices);
+    };
+  };
+}