text: Title: Feature: allow including a`faststream.broker.router.BrokerRouter()` in a `StreamRouter`
  Body: **Is your feature request related to a problem? Please describe.**
  
  Consider the following scenario: You have an existing module that's based on a BrokerRouter (e.g. a NatsRouter) and it only has subscribers and publishers. You want to be able to re-use that module in plain FastStream apps and other apps (e.g. a FastAPI app). This means that at the top-level, you need to do something like 
  
  ```
  app_router = faststream.nats.fastapi.NatsRouter() # Any subclass of StreamRouter
  app_router.include_router(shared_broker_routes.router) # This raises because `BrokerRouter.routes` doesn't exist.
  app_router.include_router(shared_api_routes.router)
  app = FastAPI()
  # ...
  ```
  
  **Describe the solution you'd like**
  It would be nice to not have to change existing BrokerRouter routes
  
  a few options might be
  - change `StreamRouter.include_router` to conditionally call `super().include_router()`
  - change BrokerRouter to expose an empty routes property (this would work for Nats + FastAPI / Starlette but I'm not sure how universal that would be.
  - Attempt to unify the concepts of BrokerRouter handlers and and APIRouter routes 
  
  **Describe alternatives you've considered**
  I currently can't directly share the code, I have to force everything to be a fastapi StreamRouter.
  