Add ability to exclude list values from indexes

Hi,

I have already asked this in the issue #3619 , but as I need an answer that I cannot find elsewhere, I open a new issue. Sorry in advance for asking this here again.

How is that supposed to be done to prevent this error `Exclude from indexes cannot be set on a list value`? Am I supposed to do the same thing than embedded entities? Something like

```
$ds = new DatastoreClient;

$data = [1, 2, 3];
$array = $ds->entity(null, $data, [
'excludeFromIndexes' => array_keys($data),
]);
```
I have tried this, and it doesn't work, so I'm wondering if there is any solution to exclude an entire list from indexes.

Thanks in advance