create SMT to add the kafkaPublishTimestamp field to outgoing message

Author: hernandezc1

broker/setup_broker/lvk/templates/ps_lvk_add_top_level_fields_smt.yaml

@@ -0,0 +1,18 @@
+# https://cloud.google.com/pubsub/docs/smts/create-topic-smt#create
+- javascriptUdf:
+    code: >
+        function addTopLevelFields(message, metadata) {
+          const data = JSON.parse(message.data);
+          const attrs = message.attributes || {};
+
+          const payload = {
+            ...data, // spread the original JSON fields into the root
+            kafkaPublishTimestamp: attrs["kafka.timestamp"] ? Number(attrs["kafka.timestamp"]) * 1000 : null
+          };
+
+          return {
+            data: JSON.stringify(payload),
+            attributes: attrs // preserve attributes
+          };
+        }
+    functionName: addTopLevelFields