[FEATURE] implemented API for shippings + created Tests

__Tasks:__
- [x] implement shippings API for GET / POST / PUT / DELETE
- [x] implement manager for saving and deleting shippings
- [x] gather feedback for my changes

__Informations:__

| Q             | A
| ------------- | ---
| Tests pass?   | aye
| Fixed tickets | -
| BC Breaks     | -
| Doc           | -