Add asgi-csrf

[florimondmanca]

Checklist

• This project is explicitly related to ASGI.
• The new list entry contains a project name, URL and description.

What is this project?

asgi-csrf is @simonw's CSRF-for-ASGI implementation. It is widely used in the Datasette ecosystem I believe: https://github.com/simonw/asgi-csrf/network/dependents

https://github.com/simonw/asgi-csrf

Do you know about other similar projects?

Yes - there's the CSRFMiddleware already listed in this list.

If so, how is this one different?

asgi-csrf is standalone, while the CSRFMiddleware we have is part of a wider library (Piccolo API). Makes me think we might want to put asgi-csrf as the lone entry later, maybe.

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