Feature: allow including afaststream.broker.router.BrokerRouter() in a StreamRouter

[strangemonad]

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.

[Lancetnik]

Thanks for the Idea!

But, I am not sure, that is required due Stream Routers' can include each over. But it is a great way to reuse your regular FastStream routers in Fast API application... I need to think and take a look on implementation. Added to backlog, but it is still feature to discuss.