fix(medusa): Performance improvement for variant availability (#3921)

**What**
- refactor availability invocations to only do one call to the inventory module

@adrien2p would love to run through some tests of this with you and get your take on approach

integration-tests/plugins/__tests__/inventory/products/list-variants.js

@@ -91,6 +91,7 @@ describe("List Variants", () => {
         stocked_quantity: 10,
       })
     })
+
     it("Decorates inventory quantities when listing variants", async () => {
       const api = useApi()