Minor improvements

- Index arrays instead of converting them into strings with separator.
- Add data prefix to retrieve Algolia search specific data for the entity.

Example of using prefixes:
1. Add 'catalog_category_load_after' event.
2. $category->setAlgoliaSpecialAttributes(array_map('trim', explode(',', $category->getSpecialAttributes())));
So "algolia_special_attributes" category data will be used instead of "special_attributes".