feat(core-flows,modules-sdk,types,medusa,link-modules): adds variant <> inventory item link endpoints (#7576)

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
```

packages/core/modules-sdk/src/utils/convert-data-to-link-definition.ts

@@ -0,0 +1,37 @@
+import { LoadedModule } from "@medusajs/types"
+import { isPresent } from "@medusajs/utils"
+import { LinkDefinition } from "../remote-link"
+
+export const convertRecordsToLinkDefinition = (
+  links: object[],
+  service: LoadedModule
+): LinkDefinition[] => {
+  const linkRelations = service.__joinerConfig.relationships || []
+  const linkDataFields = service.__joinerConfig.extraDataFields || []
+
+  const results: LinkDefinition[] = []
+
+  for (const link of links) {
+    const result: LinkDefinition = {}
+
+    for (const relation of linkRelations) {
+      result[relation.serviceName] = {
+        [relation.foreignKey]: link[relation.foreignKey],
+      }
+    }
+
+    const data: LinkDefinition["data"] = {}
+
+    for (const dataField of linkDataFields) {
+      data[dataField] = link[dataField]
+    }
+
+    if (isPresent(data)) {
+      result.data = data
+    }
+
+    results.push(result)
+  }
+
+  return results
+}