Multi DB support

[s-takehana]

About multi DB

https://cloud.google.com/datastore/docs/manage-databases

You can create multiple Firestore databases per project. You can use multiple databases to set up production and testing environments, to isolate customer data, and for data regionalization.

Google Cloud Client Libraries CHANGES.md

https://github.com/googleapis/google-cloud-go/blob/main/datastore/CHANGES.md#1130-2023-07-26