Add library function to read public vars

lib/vars.nix

@@ -0,0 +1,25 @@
+_: {
+  getPublicValue =
+    {
+
+      backend ? "in_repo",
+      default ? throw "getPublicValue: Public value ${machine}/${generator}/${file} not found!",
+      shared ? false,
+      generator,
+      machine,
+      file,
+      flake,
+    }:
+
+    if backend == "in_repo" then
+      let
+        path =
+          if shared then
+            "${flake}/vars/shared/${generator}/${file}/value"
+          else
+            "${flake}/vars/per-machine/${machine}/${generator}/${file}/value";
+      in
+      if builtins.pathExists path then builtins.readFile path else default
+    else
+      throw "backend ${backend} does not implement getPublicValue";
+}