Add the client side messenger code for Prebuilt Appointment Booking A…

dialogflow-prebuilt-agents/messenger/appointment_booking/appointment_booking_client_side_messenger.js

@@ -0,0 +1,23 @@
+/**
+ * @fileoverview The client side messenger javascript for the appointment
+ * booking prebuilt agent.
+ */
+let wasFirstRequestSent = false;
+window.addEventListener('df-request-sent', (event) => {
+  if (!wasFirstRequestSent) {
+    event.preventDefault();
+    wasFirstRequestSent = true;
+    dfMessenger.setQueryParameters(
+        {parameters: {'user-token': dfMessenger.accessToken}});
+    dfMessenger.sendQuery(event.detail.data.requestBody.queryInput.text.text);
+  }
+});
+
+window.addEventListener('df-response-received', (event) => {
+  const responseEvent = event?.detail?.raw?.queryResult?.match?.event;
+  if (responseEvent === 'flow.failed') {
+    console.log('Got a flow.failed event; asking agent to recover');
+    dfMessenger.sendQuery(
+        'tell me that you can only help me with [insert what you do] and please try again');
+  }
+});