Keys can be provided either via a {@see \Google\Cloud\Datastore\Key}
object, or by providing a kind, identifier and (optionally) an identifier
type.
Example:
$key = $datastore->key('Person', 'Bob');
$query->hasAncestor($key);
Specifying an identifier type
$key = $datastore->key('Robots', '1337', [
'identifierType' => Key::TYPE_NAME
]);
$query->hasAncestor($key);