Add asgi support to runserver

[Joshua Massover]

Conversation about this ticket can be found on the ​django forum

It would be great to add an asgi flag such that runserver will "just work" with ASGI. eg.

./manage.py runserver --asgi

The 4 options I've thought of are the following:

1. Update runserver in django in a way that keeps the daphne import optional.
2. Add daphne as a dependency to django and update runserver.
3. Maintain a separate django app that holds the command that integrates daphne with runserver. Reference the app in asgi section of the django documentation.
4. Add the commands to the daphne repository, perhaps allowing us to add daphne as an installed app so the command is discovered, and reference this in the asgi section of the django documentation.

I have an ​initial pr for (1). This code depends on #31594.