Implement lifespan middleware

[florimondmanca]

Provides startup/shutdown event handler registration, and an implementation in reaction to lifespan ASGI messages.

Proposed API:

from asgi_lifespan import Lifespan, LifespanMiddleware

# Standalone lifespan ASGI app.
lifespan = Lifespan()


# Decorator syntax:

@lifespan.on_event("startup")
async def startup():
    pass

@lifespan.on_event("shutdown")
async def shutdown():
    pass


# Imperative syntax:

async def extra_startup():
    pass

lifespan.add_event_handler("startup", extra_startup)


# Wrapping an app:
app: "ASGIApp"
app = LifespanMiddleware(app, lifespan=lifespan)