Add possibility to choose database name when using DatastoreOnlineStore #4113

Is your feature request related to a problem? Please describe.
When introducing Firestore in Datastore mode in place of Datastore, Google added a possibility to create multiple databases other than (default) (in Datastore there was only one database called (default)). Class DatastoreOnlineStore still doesn't support using a database with another name.

Describe the solution you'd like
I would like to have possibility to choose database name when using DatastoreOnlineStore

Describe alternatives you've considered
The alternative is using the (default) database. However, it has some disadvantages e.g. it doesn't allow fine-grained permissions for each user.

Additional context
Google Datastore client supports passing database when initializing the client.