[FEATURE] Added Controller, interfaces, fixtures and entities for product attributes

Added Controller, interfaces, fixtures and entities for product attributes:

__Tasks:__

- [x] added attributeType entity
- [x] added attributeType fixtures
- [x] added field descriptors for product attributes
- [x] implemented POST /admin/api/attributes
- [x] implemented GET /admin/api/attributes
- [x] implemented GET /admin/api/attributes/<id>
- [x] implemented DELETE /admin/api/attributes/<id>
- [x] added field descriptors for product attributes values
- [x] implemented GET /admin/api/attributes/<id>/values/
- [x] implemented PUT /admin/api/attributes/<id>/values/<id>
- [x] implemented POST /admin/api/attributes/<id>/values/
- [x] implemented DELETE /admin/api/attributes/<id>/values/<id>
- [ ] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | none
| Fixed tickets | none
| Doc           | none
For the sake of consistency. There was already an exception named MissingProductAttributeException. My new entity is named 'attribute'. So the name has to be MissingAttributeAttributeException, and vor values MissingValueAttributeException.