add item attributes to item manager

__Tasks:__
- [x] done
- [ ] got feedback

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none
| Doc           | none

please use trait instead of rest-helper:
See 
https://github.com/sulu-cmf/SuluProductBundle/blob/develop/Product/ProductManager.php#L51
for example