[FEATURE] refactor api entities with entity wrapper classes

__Tasks:__

- [x] gather feedback for my changes
- [ ] submit changes to the documentation


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none
@alexander-schranz I would also do that in a different way, maybe you can have a look at the following method: https://github.com/sulu-cmf/SuluProductBundle/blob/feature/product-formular/Api/Product.php#L358