persist request state into asgi scope extension

[adriangb]

A possible solution for #495

Not sure this is the best way to do it, it isn't particularly nice, and I can think of other ways to do it.
Some ideas:

• Create a class RequestState (maybe typing.SimpleNamespace?) and store that in the extension so we assign 1 attribute instead of replacing dict
• Cleanup around the extension mapping, e.g. using a TypedDict
• Assign relevant attributes one by one

On the other hand, I think this works for all cases (e.g. creating an HTTPConnection first then a Request or the other way around should work just fine since Request is a subclass of HTTPConnection and thus the mismatched attributes are strictly extra not overlapping)

I'm also not 100% sure this is valid ASGI, the spec is pretty brief on the extensions thing.