GONRG-1041 Indexer should support HTTP Target Cloud Tasks

Changes:
*Added request authentication incoming from Http cloud task
based on token verification
*Updated core-lib-gcp version in pom
*Added property with service acc mail, for token validation
*Updated google-cloud-datastore lib version , for reusing

provider/indexer-gcp/README.md

@@ -35,7 +35,7 @@ In order to run the service locally or remotely, you will need to have the follo
 | `GOOGLE_AUDIENCES` | ex `*****.apps.googleusercontent.com` | Client ID for getting access to cloud resources | yes | https://console.cloud.google.com/apis/credentials |
 | `GOOGLE_APPLICATION_CREDENTIALS` | ex `/path/to/directory/service-key.json` | Service account credentials, you only need this if running locally | yes | https://console.cloud.google.com/iam-admin/serviceaccounts |
 | `security.https.certificate.trust` | ex `false` | Elastic client connection uses TrustSelfSignedStrategy(), if it is 'true' | false | output of infrastructure deployment |
-
+| `indexer.que.service.mail` | ex `default@iam.gserviceaccount.com` | IndexerQue environment service account mail, required if Indexer Que deployed in cloud task mode, to validate token from it | yes | - |
 
 ### Run Locally
 Check that maven is installed: