feat(types,medusa): add inventory quantity to products endpoint (#7541)

what:

- when inventory_quantity is requested through the API, we calculate the inventory based on sales channels + stock locations and return the total available inventory.

A variant can have multiple inventory items. As an example:

Table: (variant)
  - 4 (required_quantity via link) x legs (inventory item)
  - 2 x table top

Only if all individual inventory items of a variant are available, do we mark the variant as available as a single unit. 

RESOLVES CORE-2187

packages/core/types/src/inventory/common.ts

@@ -88,6 +88,7 @@ export type InventoryItemDTO = {
   created_at: string | Date
   updated_at: string | Date
   deleted_at: string | Date | null
+  location_levels?: InventoryLevelDTO[]
 }
 
 /**
@@ -195,6 +196,7 @@ export type InventoryLevelDTO = {
   location_id: string
   stocked_quantity: number
   reserved_quantity: number
+  available_quantity: number
   incoming_quantity: number
   metadata: Record<string, unknown> | null
   created_at: string | Date