what:
When creating a variant, we can now create inventory as a part of the product and variants create endpoint.
This applies only to variants where `manage_inventory=true`. 2 cases present itself:
1. When inventory_items are present
- Link an inventory item with required_quantity to the variant
- the inventory item already needs to be present
2. When inventory_items are not present
- A default inventory item will be created
- links the created item to the variant with a default required_quantity
RESOLVES CORE-2220
e7005a0aac