feat(medusa, admin-ui, medusa-react, medusa-js): Allow toggling of manage inventory (#3435)

**What**
- Toggle manage inventory in the inventory management modal

**How**
- Create/update/remove inventory item based on if `manage_inventory` is set and if an inventory item already exists
- Move all stock location updates to when the modal is submitted
- Add create-inventory-item endpoint in the core

Fixes CORE-1196

Co-authored-by: Sebastian Rindom <7554214+srindom@users.noreply.github.com>

packages/medusa-js/src/resources/admin/inventory-item.ts

@@ -9,6 +9,8 @@ import {
   AdminInventoryItemsListWithVariantsAndLocationLevelsRes,
   AdminInventoryItemsLocationLevelsRes,
   AdminPostInventoryItemsItemLocationLevelsReq,
+  AdminPostInventoryItemsReq,
+  AdminPostInventoryItemsParams,
 } from "@medusajs/medusa"
 import { ResponsePromise } from "../../typings"
 import BaseResource from "../base"
@@ -75,6 +77,28 @@ class AdminInventoryItemsResource extends BaseResource {
     return this.client.request("DELETE", path, undefined, {}, customHeaders)
   }
 
+  /**
+   * Create an Inventory Item
+   * @experimental This feature is under development and may change in the future.
+   * To use this feature please install @medusajs/inventory
+   * @description creates an Inventory Item
+   * @returns the created Inventory Item
+   */
+  create(
+    payload: AdminPostInventoryItemsReq,
+    query?: AdminPostInventoryItemsParams,
+    customHeaders: Record<string, any> = {}
+  ): ResponsePromise<AdminInventoryItemsRes> {
+    let path = `/admin/inventory-items`
+    
+    if (query) {
+      const queryString = qs.stringify(query)
+      path += `?${queryString}`
+    }
+
+    return this.client.request("POST", path, payload, {}, customHeaders)
+  }
+
   /**
    * Retrieve a list of Inventory Items
    * @experimental This feature is under development and may change in the future.