[FEATURE] extended product manager and implemented ProductAttributeRepository

Extended product manager and implemented ProductAttributeRepository:

__Tasks:__

- [x] Extended productManager save method so that it can handle product attributes
- [x] Created ProductAttributeRepository and added methods for fetching product attributes by product   and attribute id
- [ ] gather feedback for my changes



__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none
| Doc           | none
move this inside this condition:
 $this->em->persist($productAttribute);