Relax shutdown_trigger annotation

[Tinche]

I relaxed the shutdown_trigger annotation to only take an Awaitable (a shorthand for Awaitable[Any], imo more readable and not requiring an import).

I don't think there's a reason for the trigger to be annotated as Awaitable[None] since we don't care about the return value, and it makes it not type-check with an asyncio Event, which is a common use case.

[pgjones]

Thanks