Add the sentry hub into the asgi_scope for other middlewares

[meadsteve]

With the sentry hub attached to the asgi scope then other middlwares (or request handlers) can access the sentry hub.

[stratosgear]

I think this is a nice addition, that I would definitely take advantage off. Can this get merged (and released?)

Thanks for this utility, btw!

[untitaker]

What's wrong with accessing Hub.current?