Add support for excludeFromIndexes in embedded entities

[jdpedrie]

cc @maksimru

This change adds support for excluding properties of embedded entities from datastore indexes.

use Google\Cloud\Datastore\Entity;

$e = $datastore->entity('Person', [
    'entity' => [
        'prop' => 'val',
        Entity::EXCLUDE_FROM_INDEXES => ['prop']
    ]
]);

$datastore->insert($e);

We've added a todo item for the next major version to improve on this, but for the time being this is the best option to avoid a backwards-compatibility break.