Add startup and shutdown handlers to existing FastAPI app

[falkoschindler]

This PR tries to solve #1870 and #1874 by adding startup and shutdown handlers to an existing FastAPI app.
This has been broken by switching to the new lifespan API. But the lifespan events of a sub application don't seem to be executed. Therefore this extra step is necessary.

[falkoschindler]

Test app:

test.py

from fastapi import FastAPI
from nicegui import ui

def lifespan(_):
    print("main startup")
    yield
    print("main shutdown")

app = FastAPI(lifespan=lifespan)
# or without lifespan:
# app = FastAPI()

@app.get("/")
async def root():
    return {"message": "Hello World"}

@ui.page('/page')
def page():
    ui.button('Hi!', on_click=lambda: ui.notify('Hello World!'))

ui.run_with(app)

Run with: uvicorn test:app --port 8000

[rodja]

I have not time to test it myself today :-(
Maybe someone else can do a review for me, @ProbablyBrian and/or @zilch42?

[rodja]

I just walked the code with @falkoschindler. Seems to work as expected.