[typing] Allow non-class callables to be used as middleware

[decorator-factory]

1. Right now, it's possible to pass a sequence of tuples (callable, options) where callable is a callable accepting **options and returning an ASGI app. But the type annotation on Starlette says that one has to pass a sequence of Middleware objects.
   This PR allows passing in a sequence of tuples as well as a list of Middleware objects

2. Type annotations only allow classes to be used as middleware, but it's possible to provide any callable that returns an ASGI app. This PR fixes that as well.

[lovelydinosaur]

Thanks. Honestly I'd rather prefer we don't add extra options here.
Keeping things constrained to one particular style helps keep everything just a little bit more simple.

[decorator-factory]

That's understandable. But why can't I use a function as an ASGI app? It works just fine at runtime.

[graingert]

I'm in favour of this being a Callable[[], asgiref.typing.ASGI3App]

[Kludex]

I'll go with Tom on the applications.py - too complex. But I think the cls parameter needs a change of type on the Middleware.__init__.

I'm in favour of this being a Callable[[], asgiref.typing.ASGI3App]

We don't have asgiref here yet.

I'll close this based on @tomchristie 's and my own comments, and the fact that the goal of the PR was to attack the middleware parameter (cls was just a side effect). Thanks for effort @decorator-factory .