feat: add Advanced Service samples for Chat (#481)

* feat: add Advanced Service samples for Chat

* README improvements

* README improvements #2

---------

Co-authored-by: pierrick <[email protected]>

chat/advanced-service/AppAuthenticationUtils.gs

@@ -0,0 +1,61 @@
+/**
+ * Copyright 2024 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// [START chat_authentication_utils]
+
+// This script provides configuration and helper functions for app authentication.
+// It may require modifications to work in your environment.
+
+// For more information on app authentication, see
+// https://developers.google.com/workspace/chat/authenticate-authorize-chat-app
+
+const APP_AUTH_OAUTH_SCOPES = ['https://www.googleapis.com/auth/chat.bot'];
+// Warning: This example uses a service account private key, it should always be stored in a
+// secure location.
+const SERVICE_ACCOUNT = {
+  // TODO(developer): Replace with the Google Chat credentials to use for app authentication,
+  // the service account private key's JSON.
+};
+
+/**
+ * Authenticates the app service by using the OAuth2 library.
+ *
+ * @return {Object} the authenticated app service
+ */
+function getService_() {
+  return OAuth2.createService(SERVICE_ACCOUNT.client_email)
+    .setTokenUrl(SERVICE_ACCOUNT.token_uri)
+    .setPrivateKey(SERVICE_ACCOUNT.private_key)
+    .setIssuer(SERVICE_ACCOUNT.client_email)
+    .setSubject(SERVICE_ACCOUNT.client_email)
+    .setScope(APP_AUTH_OAUTH_SCOPES)
+    .setCache(CacheService.getUserCache())
+    .setLock(LockService.getUserLock())
+    .setPropertyStore(PropertiesService.getScriptProperties());
+}
+
+/**
+ * Generates headers with the app credentials to use to make Google Chat API calls.
+ *
+ * @return {Object} the header with credentials
+ */
+function getHeaderWithAppCredentials() {
+  return {
+    'Authorization': `Bearer ${getService_().getAccessToken()}`
+  };
+}
+
+// [END chat_authentication_utils]