Implement ASGI extra http.response.pathsend

[gi0baro]

As suggested by @Kludex

ASGI protocol might take advantage of granian capability of directly serve files from a path, like already implemented in RSGI proto.response_file.

This would require to open up a PR to https://github.com/django/asgiref adding the new extra, which might look like the following:

"scope": {
    ...
    "extensions": {
        "http.response.file_path": {},
    },
}

With the above extra definition, a new event might be produced by ASGI applications, with the following keys:

• type (Unicode string): "http.response.file_path"
• file_path (Unicode string): The string representation of the file path to serve (platform specific)

The ASGI application is still responsible to send the http.response.start event with the relevant headers (eg: content-type, content-length, etc.)