Fetch GCP-Datastore entity by id inside of a Dataflow transform

I have a dataflow pipeline where first I queried one model, but then for each record that pointed to another model I wanted to be able to fetch that too.

Browsing the `apache_beam & google.cloud.proto.datastore.v1 `source I was able to make this happen:

[https://stackoverflow.com/a/55616668/4458510](https://stackoverflow.com/a/55616668/4458510)

I copied the same mechanism from the other Datastore DoFn's and it seems to work fine, and I think something like this would be a good addition to apache_beam.io.gcp.datastore.v1.datastoreio in the sdk

Imported from Jira [BEAM-7048](https://issues.apache.org/jira/browse/BEAM-7048). Original Jira may contain additional context.
Reported by: alexindaco.