add support for lifespan tasks

[Lendemor]

Add support for running task during the whole lifespan of the app, rather than linked to any specific state.

import asyncio
import reflex as rx

@rx.page()
def index():
    return rx.box(), rx.color_mode.button(position="top-right")

async def long_running_task(key: str = "A"):
    i = 1
    while True:
        await asyncio.sleep(5)
        print(f"{key}: Oh I woke up {i} times.... Back to sleep again")
        i += 1


async def longer_running_task():
    while True:
        await asyncio.sleep(10)
        print("I sleep more than the others.")

@contextlib.asynccontextmanager
async def my_custom_context():
    print("Startup process....")
    yield
    print("Shutdown process....")

app = rx.App()
app.register_lifespan_task(long_running_task)
app.register_lifespan_task(long_running_task, "B")
app.register_lifespan_task(longer_running_task)
app.register_lifespan_task(my_custom_context)

[Lendemor]

The tasks should have a chance to clean up nicely without being cancelled right?

I think it's up to the task itself to handle being cancelled cleanly, by using try/finally or catching the asyncio.CancelledError