Add implementation for Google Cloud Datastore

[philippgille]

Amazon DynamoDB and Azure Table Storage are already supported, now GCP is missing for support of the "big three" cloud providers.

• Info: https://cloud.google.com/datastore/
• GoDoc: https://godoc.org/cloud.google.com/go/datastore
• Emulator: https://cloud.google.com/datastore/docs/tools/datastore-emulator
  • Maybe it can be run via the official GCP SDK Docker image: https://github.com/GoogleCloudPlatform/cloud-sdk-docker
  • Otherwise: https://github.com/SingularitiesCR/datastore-emulator-docker
• Check if the SDK uses gRPC, otherwise maybe use the gRPC interface directly: https://cloud.google.com/datastore/docs/reference/data/rpc/

Note: Cloud Firestore (not Firebase) has a "Datastore mode" which makes it compatible to the Datastore API. Firestore might superseed Datastore in the future. But it's currently marked as beta:

• https://cloud.google.com/firestore/
• https://cloud.google.com/datastore/docs/firestore-or-datastore