Improved wsgi middleware, mainly a copy of a2wsgi

[euri10]

Should fix #371
I added types as well

[Kludex]

I guess we only need to remove the types on the wsgi.py file and rebase?

[abersheeran]

I think it might be more convenient to directly use a2wsgi as a dependency. As far as I know, Uvicorn and Starlette both have WSGIMiddleware, which is a duplicate. And it will cause the changes in one of the items to not be quickly reflected in the other item.

[euri10]

that's indeed a good idea, my only concern is the types, not sure how that would work since we're not using the same base

[Kludex]

We probably should:

1. Sync with Starlette. Should we only remove from uvicorn or removing on starlette also makes sense?
2. If we want to remove it from uvicorn only, add a DeprecationWarning, recommending a2wsgi. Then later on we can remove it.
3. Update documentation.

[Flauschbaellchen]

Hi,

I run into the same problem as described in #371 as my app needs to support uploads of large files which are often 1GB or much bigger.
I would like to support here as much as I can. Do you need someone for testing purposes?