datastoreio discards entity key

datastoreio discards the entity key and just returns a collection of decoded entities.

This might work for some use cases but often you need to know the key of the entity you're operating on, e.g. if you need to update it or store a reference to it.

Maybe it should return a PCollection<*datastore.Key, T\> instead of just PCollection?

Or PCollection using key.Encode(), since the go sdk doesn't seem to support pointers yet.

Imported from Jira [BEAM-5201](https://issues.apache.org/jira/browse/BEAM-5201). Original Jira may contain additional context.
Reported by: ribrdb.