what:
- adds variant inventory link management endpoints:
```
Link inventory item to variant
POST /products/:id/variants/:vid/inventory-items
Update variant's inventory item link
POST /products/:id/variants/:vid/inventory-items/:iid
Unlink variant's inventory item
DELETE /products/:id/variants/:vid/inventory-items/:iid
```
- a batch endpoint that does the above 3 across variants
```
POST /products/:id/variants/inventory-items
```
**what**
- Create return workflow partial implementation
- Update some order domain types
- create order fulfillment link
**NOTE**
this PR is partially done but can still be merged as is, it will require some discussions around the flow and some unknowns or uncertainty in regards to some data and some behaviour
