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-react/mocks/handlers/admin.ts

@@ -1705,6 +1705,15 @@ export const adminHandlers = [
     )
   }),
 
+  rest.post("/admin/order-edits/:id/items", (req, res, ctx) => {
+    return res(
+      ctx.status(200),
+      ctx.json({
+        order_edit: { ...fixtures.get("order_edit"), ...(req.body as any) },
+      })
+    )
+  }),
+
   rest.post("/admin/order-edits/:id/request", (req, res, ctx) => {
     return res(
       ctx.status(200),

packages/medusa-react/src/hooks/admin/order-edits/mutations.ts

@@ -8,6 +8,7 @@ import {
   AdminPostOrderEditsEditLineItemsLineItemReq,
   AdminPostOrderEditsOrderEditReq,
   AdminPostOrderEditsReq,
+  AdminPostOrderEditsEditLineItemsReq,
 } from "@medusajs/medusa"
 
 import { buildOptions } from "../../utils/buildOptions"
@@ -114,6 +115,27 @@ export const useAdminUpdateOrderEdit = (
   )
 }
 
+export const useAdminOrderEditLineItem = (
+  id: string,
+  options?: UseMutationOptions<
+    Response<AdminOrderEditsRes>,
+    Error,
+    AdminPostOrderEditsEditLineItemsReq
+  >
+) => {
+  const { client } = useMedusa()
+  const queryClient = useQueryClient()
+  return useMutation(
+    (payload: AdminPostOrderEditsEditLineItemsReq) =>
+      client.admin.orderEdits.addLineItem(id, payload),
+    buildOptions(
+      queryClient,
+      [adminOrderEditsKeys.lists(), adminOrderEditsKeys.detail(id)],
+      options
+    )
+  )
+}
+
 export const useAdminRequestOrderEditConfirmation = (
   id: string,
   options?: UseMutationOptions<Response<AdminOrderEditsRes>, Error>

packages/medusa-react/test/hooks/admin/order-edits/mutations.test.ts

@@ -6,6 +6,8 @@ import {
   useAdminDeleteOrderEditItemChange,
   useAdminOrderEditUpdateLineItem,
   useAdminRequestOrderEditConfirmation,
+  useAdminOrderEditLineItem,
+  useAdminCancelOrderEdit,
   useAdminUpdateOrderEdit,
 } from "../../../../src/"
 import { fixtures } from "../../../../mocks/data"
@@ -163,6 +165,36 @@ describe("useAdminRequestOrderEditConfirmation hook", () => {
   })
 })
 
+describe("useAdminOrderEditLineItem hook", () => {
+  test("Created an order edit line item", async () => {
+    const { result, waitFor } = renderHook(
+      () => useAdminOrderEditLineItem(fixtures.get("order_edit").id),
+      {
+        wrapper: createWrapper(),
+      }
+    )
+
+    const payload = {
+      variant_id: "var_1",
+      quantity: 2,
+    }
+
+    result.current.mutate(payload)
+
+    await waitFor(() => result.current.isSuccess)
+
+    expect(result.current.data.response.status).toEqual(200)
+    expect(result.current.data).toEqual(
+      expect.objectContaining({
+        order_edit: {
+          ...fixtures.get("order_edit"),
+          ...payload,
+        },
+      })
+    )
+  })
+})
+
 describe("useAdminCancelOrderEdit hook", () => {
   test("cancel an order edit", async () => {
     const { result, waitFor } = renderHook(