datastore: add support for scatter query

**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.