feat: Region PaymentProvider link (#6577)

**What**

- Introduce link between Region and PaymentProvider
- Introduce API endpoint `GET /store/regions/:id/payment-providers` for retrieving providers by region
- Add tests for both

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

@@ -10,3 +10,5 @@ export * from "./product-shipping-profile"
 export * from "./product-variant-inventory-item"
 export * from "./product-variant-price-set"
 export * from "./publishable-api-key-sales-channel"
+export * from "./region-payment-provider"
+

packages/link-modules/src/definitions/region-payment-provider.ts

@@ -0,0 +1,64 @@
+import { Modules } from "@medusajs/modules-sdk"
+import { ModuleJoinerConfig } from "@medusajs/types"
+import { LINKS } from "../links"
+
+export const RegionPaymentProvider: ModuleJoinerConfig = {
+  serviceName: LINKS.RegionPaymentProvider,
+  isLink: true,
+  databaseConfig: {
+    tableName: "region_payment_provider",
+    idPrefix: "regpp",
+  },
+  alias: [
+    {
+      name: ["region_payment_provider", "region_payment_providers"],
+      args: {
+        entity: "LinkRegionPaymentProvider",
+      },
+    },
+  ],
+  primaryKeys: ["id", "region_id", "payment_provider_id"],
+  relationships: [
+    {
+      serviceName: Modules.REGION,
+      primaryKey: "id",
+      foreignKey: "region_id",
+      alias: "region",
+    },
+    {
+      serviceName: Modules.PAYMENT,
+      primaryKey: "id",
+      foreignKey: "payment_provider_id",
+      alias: "payment_provider",
+      args: { methodSuffix: "PaymentProviders" },
+    },
+  ],
+  extends: [
+    {
+      serviceName: Modules.REGION,
+      fieldAlias: {
+        payment_providers: "payment_provider_link.payment_provider",
+      },
+      relationship: {
+        serviceName: LINKS.RegionPaymentProvider,
+        primaryKey: "region_id",
+        foreignKey: "id",
+        alias: "payment_provider_link",
+        isList: true,
+      },
+    },
+    {
+      serviceName: Modules.PAYMENT,
+      fieldAlias: {
+        regions: "region_link.region",
+      },
+      relationship: {
+        serviceName: LINKS.RegionPaymentProvider,
+        primaryKey: "payment_provider_id",
+        foreignKey: "id",
+        alias: "region_link",
+        isList: true,
+      },
+    },
+  ],
+}

packages/link-modules/src/links.ts

@@ -20,6 +20,12 @@ export const LINKS = {
     Modules.PAYMENT,
     "payment_collection_id"
   ),
+  RegionPaymentProvider: composeLinkName(
+    Modules.REGION,
+    "region_id",
+    Modules.PAYMENT,
+    "payment_provider_id"
+  ),
   CartPromotion: composeLinkName(
     Modules.CART,
     "cart_id",