Add http backend

[almarklein]

A backend that runs a webserver that you can connect to with your browser. Multiple clients are supported (eventually).

Using ASGI, so it can run on any ASGI server and we don't have any hard dependencies 🚀

• ASGI server.
• Websocket connection.
• Implement JS to be a AFM (anywidget front-end module) host. Or JS to control the renderview directly.
• Python backend module.
• Support for multiple simultaneous clients.
• How to include this in a larger web app, e.g. FastAPI / Falcon etc. Create an example.