feat: add a line item to an order edit (#2243)

**What**
- Implement adding a line item to order (edit)

**How**
- _by implementing the following "flow"_
  - generate a line item
  - computing line item adjustments for that line item
  - creating tax lines
  - creating a change record

**Testing**
- **_integration tests_**
  - check if line item and order item change objects are created (with correct tax lines)
  - line item adjustments are generated if
    - fixed discount is applied to cart
    - percentage discount is applied
- **_unit tests_** 
  - ensure that methods from Inventory, LineItem, LineItemAdjustment etc. services are called 

---

RESOLVES CORE-495

packages/medusa-js/src/resources/admin/order-edits.ts

@@ -2,9 +2,10 @@ import {
   AdminOrderEditDeleteRes,
   AdminOrderEditItemChangeDeleteRes,
   AdminOrderEditsRes,
-  AdminPostOrderEditsEditLineItemsLineItemReq,
   AdminPostOrderEditsOrderEditReq,
   AdminPostOrderEditsReq,
+  AdminPostOrderEditsEditLineItemsReq,
+  AdminPostOrderEditsEditLineItemsLineItemReq,
 } from "@medusajs/medusa"
 import { ResponsePromise } from "../../typings"
 import BaseResource from "../base"
@@ -43,6 +44,15 @@ class AdminOrderEditsResource extends BaseResource {
     return this.client.request("DELETE", path, undefined, {}, customHeaders)
   }
 
+  addLineItem(
+    id: string,
+    payload: AdminPostOrderEditsEditLineItemsReq,
+    customHeaders: Record<string, any> = {}
+  ): ResponsePromise<AdminOrderEditsRes> {
+    const path = `/admin/order-edits/${id}/items`
+    return this.client.request("POST", path, payload, {}, customHeaders)
+  }
+
   deleteItemChange(
     orderEditId: string,
     itemChangeId: string,