Support retrieving and editing excludeFromIndexes (parity with other client libraries)

[calsmith]

Is your feature request related to a problem? Please describe.
There is no way to get/set which fields of an entity are indexed after initial entity creation.

Use case:

1. Retrieve an entity
2. Add a new field to the entity
3. Exclude the new field from being indexed

Unlike the other client libraries, there is no way to achieve this with Node JS.

Describe the solution you'd like
Here's example code from the PHP Datastore library which easily enables this use case:

$ds = Datastore::get();
$entity = $ds->lookup($ds->key(...));
$entity["newField"] = 123;
$unindexedFields = $entity->excludedProperties(); // Get the unindexed fields.
$unindexedFields[] = "newField"; // Prevent the new field from being indexed.
$entity->setExcludeFromIndexes($unindexedFields);
$ds->update($entity);

Describe alternatives you've considered
Without this functionality, we cannot use Node JS or this library for our project.

Additional context
Related requests from other customers:
#914
#935

[danieljbruce]

To support this feature we should write code to create and delete indexes. The following link shows this is not supported yet.

nodejs-datastore/src/index-class.ts

Lines 150 to 151 in cd59c71

	// @TODO implement create()
	// @TODO implement delete()