feat(medusa): Performance improvements of Carts domain (#2648)

**What**

I have created a new method on the cart service which is `addLineItems`, allowing a user to add one or multiple items in an optimized way. Also updated the `generate` method from the line item service which now also accept a object data or a collection of data which. Various places have been optimized and cache support has been added to the price selection strategy.

The overall optimization allows to reach another 9000% improvement in the response time as a median (Creating a cart with 6 items):

|   | Min (ms)  | Median (ms)  | Max (ms)  | Median Improvement (%)
|---|:-:|---|---|---|
| Before optimisation  | 1200  | 9999 | 12698  |  N/A
| After optimisation | 63  | 252  | 500  | 39x
| After re optimisation | 56 | 82  | 399  | 121x
| After including addressed feedback | 65 | 202  | 495  | 49x

FIXES CORE-722

packages/medusa/src/services/__tests__/discount.js

@@ -1,6 +1,8 @@
 import { IdMap, MockManager, MockRepository } from "medusa-test-utils"
 import { FlagRouter } from "../../utils/flag-router"
 import DiscountService from "../discount"
+import { TotalsServiceMock } from "../__mocks__/totals"
+import { newTotalsServiceMock } from "../__mocks__/new-totals"
 
 const featureFlagRouter = new FlagRouter({})
 
@@ -601,15 +603,28 @@ describe("DiscountService", () => {
     })
 
     const totalsService = {
-      getSubtotal: () => {
+      ...TotalsServiceMock,
+      getSubtotal: async () => {
         return 1100
       },
     }
 
+    const newTotalsService = {
+      ...newTotalsServiceMock,
+      getLineItemTotals: async () => {
+        return [
+          {
+            subtotal: 1100,
+          },
+        ]
+      },
+    }
+
     const discountService = new DiscountService({
       manager: MockManager,
       discountRepository,
       totalsService,
+      newTotalsService,
       featureFlagRouter,
     })
 
@@ -631,21 +646,31 @@ describe("DiscountService", () => {
     })
 
     it("correctly calculates fixed + total discount", async () => {
+      let item = {
+        unit_price: 400,
+        quantity: 2,
+        allow_discounts: true,
+      }
+
       const adjustment1 = await discountService.calculateDiscountForLineItem(
         "disc_fixed_total",
+        item,
         {
-          unit_price: 400,
-          quantity: 2,
-          allow_discounts: true,
+          items: [item],
         }
       )
 
+      item = {
+        unit_price: 300,
+        quantity: 1,
+        allow_discounts: true,
+      }
+
       const adjustment2 = await discountService.calculateDiscountForLineItem(
         "disc_fixed_total",
+        item,
         {
-          unit_price: 300,
-          quantity: 1,
-          allow_discounts: true,
+          items: [item],
         }
       )