feature(api): allow convenience methods to return ElggBatch as a result

Fixes #6676 - allows developers to pass a flag in the options to return an ElggBatch from convenience methods when they need to iterate through large sets

TODO
- [ ] Tests
I think we could boil this logic to:

```php
$batch = ElggBatch::checkOptions($options, 'elgg_get_annotations');
if ($batch) {
    return $batch;
}
```