feat: add multibrokers support

[Lancetnik]

Adds multibroker support across the application stack (internal application, FastStream app, ASGI integration) and updates related tests.

from faststream import FastStream, Logger
from faststream.nats import NatsBroker
from faststream.rabbit import RabbitBroker, RabbitResponse

rbroker = RabbitBroker()

@rbroker.subscriber("test")
async def handler_rmq(msg: str, logger: Logger) -> None:
    logger.info(msg)

nbroker = NatsBroker()

@nbroker.subscriber("test2")
@rbroker.publisher("test")
async def handler_nats(msg: str, logger: Logger):
    logger.info(msg)
    return RabbitResponse(msg)

app = FastStream(rbroker, nbroker)

@app.after_startup
async def _() -> None:
    await nbroker.publish("Hi!", "test2")


# testing
import pytest

from faststream import TestApp
from faststream.nats import TestNatsBroker
from faststream.rabbit import TestRabbitBroker

@pytest.mark.asyncio()
async def test_smoke():
    async with TestApp(app), TestNatsBroker(nbroker) as nbr, TestRabbitBroker(rbroker):
        await nbr.publish("Hi!", "test2")
        await handler_rmq.wait_call(3)
        handler_rmq.mock.assert_called_once_with("Hi!")

Changed files: faststream/_internal/application.py, faststream/app.py, faststream/asgi/app.py, plus tests in tests/application/test_delayed_broker.py, tests/asgi/testcase.py, and tests/cli/require_broker/test_app.py.

Closes #526