New method to get child categories based on parent category id

[FabDeep]

In order to get all upcoming events based on a category that has child categories, we need to request the parent category ID and get all child category IDs in return. Additionally, we must be able to get all upcoming events with a combination of parameters (e.g. category, location and event type).

Since we need to use $client->event->findUpcoming(); in this particular use case, we can't rely on something like findByCategory(int $id, bool $includeChildren);

Instead, we need a method that gets passed the ID of a parent category and returns the parent category ID with all its child category IDs in a comma separated string.

e.g. $categoryIds = $client->category->getChildCategories(int $id);