Advanced typing of ASGI `scope` dict

Create a more advanced `TypedDict` or similar for annotating ASGI `scope`. We could probably draw inspiration from the relevant parts of `asgiref`: https://github.com/django/asgiref/blob/main/asgiref/typing.py. Since these annotations are really trivial, we probably want just to reimplement the part that we use. This would allow us to get rid of a bunch of typing ignores, see #2599.