feat(core-flows,medusa): adds inventory kit creation to variants endpoint (#7599)

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

packages/medusa/src/api/store/products/helpers.ts

@@ -87,8 +87,8 @@ export const wrapVariantsWithInventoryQuantity = async (
 
     for (const link of links) {
       const requiredQuantity = link.required_quantity
-      const availableQuantity = (link.inventory.location_levels || []).reduce(
-        (sum, level) => sum + level.available_quantity || 0,
+      const availableQuantity = (link.inventory?.location_levels || []).reduce(
+        (sum, level) => sum + (level?.available_quantity || 0),
         0
       )