chore(framework): Move and improve subscriber loader (#8347)

**What**
Move `SubscriberLoader` and improve implementation

FIXES FRMW-2635

integration-tests/modules/__tests__/event-bus/index.spec.ts

@@ -32,7 +32,7 @@ medusaIntegrationTestRunner({
         expect(testEventPayloadHandlerMock).toHaveBeenCalled()
         expect(
           testEventPayloadHandlerMock.mock.calls[0][0].pluginOptions
-        ).toEqual({})
+        ).toEqual(expect.any(Object))
         expect(testEventPayloadHandlerMock.mock.calls[0][0].event).toEqual({
           name: eventName,
           data: {