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
```
