datastore: add support for scatter query

[maros7]

Is your feature request related to a problem? Please describe. When querying a large kind it is done sequentially and is taking a long time.

Describe the solution you'd like A similar solution as in the Java library. Reference: https://github.com/googleapis/google-cloud-datastore/blob/6ea7ca0ad2a278bc2e903a5a89a6cacc35e90b57/java/datastore/src/main/java/com/google/datastore/v1/client/QuerySplitter.java#L26 https://github.com/googleapis/java-datastore/issues/294#issuecomment-756449920

Describe alternatives you've considered We have implemented it on our own. But I guess that other people using this library might find it useful.

Additional context Add any other context or screenshots about the feature request here.