feat: Add the basic implementation of notification module (#7282)

* feat: Add the basic implementation of notification module

* fix: Minor fixes and introduction of idempotency key

* fix: Changes based on PR review

packages/modules/notification/integration-tests/__fixtures__/providers/default-provider.ts

@@ -0,0 +1,14 @@
+import { NotificationTypes } from "@medusajs/types"
+import { AbstractNotificationProviderService } from "@medusajs/utils/src"
+
+export class NotificationProviderServiceFixtures extends AbstractNotificationProviderService {
+  static identifier = "fixtures-notification-provider"
+
+  async send(
+    notification: NotificationTypes.ProviderSendNotificationDTO
+  ): Promise<NotificationTypes.ProviderSendNotificationResultsDTO> {
+    return { id: "external_id" }
+  }
+}
+
+export const services = [NotificationProviderServiceFixtures]

packages/modules/notification/integration-tests/__fixtures__/providers/index.ts

@@ -0,0 +1 @@
+export * from "./default-provider"