feat: Added node sample codes for cloud-iot-token-service generateAccessToken API

samples/access_token_example/.gitignore

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+resources/*

samples/access_token_example/README.md

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+<img src="https://avatars2.githubusercontent.com/u/2810941?v=3&s=96" alt="Google Cloud Platform logo" title="Google Cloud Platform" align="right" height="96" width="96"/>
+
+# Google Cloud IoT Core NodeJS Device Federated Authentication example
+
+This sample app demonstrates the capabilites of Google Cloud IoT Core device federated authentication feature.
+
+Devices authenticated to Cloud IoT Core can use the [Token Service](https://cloud.google.com/iot/alpha/docs/reference/cloudiottoken/rest) federated authentication to request [OAuth 2.0 access tokens](https://developers.google.com/identity/protocols/oauth2) in exchange for their [Cloud IoT Core JWTs](https://cloud.google.com/iot/docs/how-tos/credentials/jwts).
+
+The OAuth 2.0 credentials can be used to call different [Google Cloud APIs](https://developers.google.com/identity/protocols/oauth2/scopes) with fine-grained permissions and access control using [Workload Identity Federation](https://cloud.google.com/iam/docs/workload-identity-federation).
+
+For more information, see https://cloud.google.com/iot/alpha/docs/how-tos/federated_auth
+
+# Setup
+
+Run the following command to install the library dependencies for NodeJS:
+
+    npm install
+
+# Running the sample
+
+    Commands:
+      generateAccessToken                                 Generates OAuth 2.0 Google Access Token.
+      exchangeDeviceAccessTokenToServiceAccountToken      Exchanges device access token to service account access token.
+      publishPubSubMessage                                Publishes a message to Cloud Pub/Sub topic.
+      downloadCloudStorageFile                            Downloads a file from Cloud Storage bucket.
+      sendCommandToIoTDevice                              Sends a command to an IoT device.
+
+    Examples:
+      node access_token.js generateAccessToken --cloudRegion=us-central1 --projectId=my-project my-registry my-device https://www.googleapis.com/auth/cloud-platform RS256 ../resources/rsa_private.pem
+      node access_token.js publishPubSubMessage --cloudRegion=us-central1 --projectId=my-project my-registry my-device RS256 ../resources/rsa_private.pem my-pubsub-topic
+      node access_token.js downloadCloudStorageFile --cloudRegion=us-central1 --projectId=my-project my-registry my-device RS256 ../resources/rsa_private.pem my-storage-bucket ../resources/logo.png
+      node access_token.js sendCommandToIoTDevice --cloudRegion=us-central1 --projectId=my-project my-registry my-device RS256 ../resources/rsa_private.pem my-service-account@my-project.iam.gserviceaccount.com
+      node access_token.js exchangeDeviceAccessTokenToServiceAccountToken device-access-token my-service-account@my-project.iam.gserviceaccount.com
+
+For more information, see https://cloud.google.com/iot-core/docs