feat(core-flows,link-modules,modules-sdk): add cart <> promotion link as source of truth (#6561)

what:

- adds promotion cart link
- update steps to create and remove links

packages/link-modules/src/definitions/cart-promotion.ts

@@ -0,0 +1,55 @@
+import { Modules } from "@medusajs/modules-sdk"
+import { ModuleJoinerConfig } from "@medusajs/types"
+import { LINKS } from "../links"
+
+export const CartPromotion: ModuleJoinerConfig = {
+  serviceName: LINKS.CartPromotion,
+  isLink: true,
+  databaseConfig: {
+    tableName: "cart_promotion",
+    idPrefix: "cartpromo",
+  },
+  alias: [
+    {
+      name: ["cart_promotion", "cart_promotions"],
+      args: {
+        entity: "LinkCartPromotion",
+      },
+    },
+  ],
+  primaryKeys: ["id", "cart_id", "promotion_id"],
+  relationships: [
+    {
+      serviceName: Modules.CART,
+      primaryKey: "id",
+      foreignKey: "cart_id",
+      alias: "cart",
+    },
+    {
+      serviceName: Modules.PROMOTION,
+      primaryKey: "id",
+      foreignKey: "promotion_id",
+      alias: "promotion",
+    },
+  ],
+  extends: [
+    {
+      serviceName: Modules.CART,
+      relationship: {
+        serviceName: LINKS.CartPromotion,
+        primaryKey: "cart_id",
+        foreignKey: "id",
+        alias: "cart_link",
+      },
+    },
+    {
+      serviceName: Modules.PROMOTION,
+      relationship: {
+        serviceName: LINKS.CartPromotion,
+        primaryKey: "promotion_id",
+        foreignKey: "id",
+        alias: "promotion_link",
+      },
+    },
+  ],
+}

packages/link-modules/src/definitions/index.ts

@@ -1,5 +1,6 @@
 export * from "./cart-customer"
 export * from "./cart-payment-collection"
+export * from "./cart-promotion"
 export * from "./cart-region"
 export * from "./cart-sales-channel"
 export * from "./inventory-level-stock-location"

packages/link-modules/src/links.ts

@@ -20,6 +20,12 @@ export const LINKS = {
     Modules.PAYMENT,
     "payment_collection_id"
   ),
+  CartPromotion: composeLinkName(
+    Modules.CART,
+    "cart_id",
+    Modules.PROMOTION,
+    "promotion_id"
+  ),
 
   // Internal services
   ProductShippingProfile: composeLinkName(