Key only reads from Google Datastore

Currently there is no functionality allowing to read only keys from the Google Datastore through the Datastore IO. In some cases users don't need to read the whole entity, e.g. to filter by certain values in ancestry. This seems to be an important feature as the entity reads are expensive and thus the native Datastore client/API allow to run Key Only queries.

Imported from Jira [BEAM-2819](https://issues.apache.org/jira/browse/BEAM-2819). Original Jira may contain additional context.
Reported by: ilnar.

Start by locating the Google Datastore IO and its query construction, then review how the native Datastore API represents Key Only queries. Add a way for reads to request keys without full entities, and add coverage showing that the resulting query returns keys for ancestry filtering.

Written by the indexing model from the issue text.