feat(index): Index module foundation (#9095)

**What**
Index module foundation

Co-authored-by: Carlos R. L. Rodrigues <37986729+carlos-r-l-rodrigues@users.noreply.github.com>

packages/modules/index/integration-tests/__fixtures__/event-bus.ts

@@ -0,0 +1,47 @@
+import {
+  EventBusTypes,
+  IEventBusModuleService,
+  Message,
+  Subscriber,
+} from "@medusajs/types"
+
+export class EventBusServiceMock implements IEventBusModuleService {
+  protected readonly subscribers_: Map<string | symbol, Set<Subscriber>> =
+    new Map()
+
+  async emit<T>(
+    messages: Message<T> | Message<T>[],
+    options?: Record<string, unknown>
+  ): Promise<void> {
+    const messages_ = Array.isArray(messages) ? messages : [messages]
+
+    for (const message of messages_) {
+      const subscribers = this.subscribers_.get(message.name)
+
+      for (const subscriber of subscribers ?? []) {
+        const { options, ...payload } = message
+        await subscriber(payload)
+      }
+    }
+  }
+
+  subscribe(event: string | symbol, subscriber: Subscriber): this {
+    this.subscribers_.set(event, new Set([subscriber]))
+    return this
+  }
+
+  unsubscribe(
+    event: string | symbol,
+    subscriber: Subscriber,
+    context?: EventBusTypes.SubscriberContext
+  ): this {
+    return this
+  }
+
+  releaseGroupedEvents(eventGroupId: string): Promise<void> {
+    throw new Error("Method not implemented.")
+  }
+  clearGroupedEvents(eventGroupId: string): Promise<void> {
+    throw new Error("Method not implemented.")
+  }
+}