Allow using built-in process manager with one worker

[eltoder]

Uvicorn has a built-in process manager, so I can run it with, for example, --workers 4 to get 4 worker processes. Uvicorn will run 4 child processes listening on the same port, and will automatically restart processes that die or get stuck. However, if I use --workers 1, Uvicorn will not use its process manager and will instead run the ASGI application directly in the main process.

I have a use case where I only need one worker process, but I need to monitor and restart it on failures. I can use an external process manager like gunicorn or supervisor, but it seems unfortunate to introduce another dependency when everything I need is already supported by Uvicorn itself. The built-in process manager can already run with just one worker by starting with --workers 2 and then sending SIGTTOU. Alternatively, I can copy code form main.py and use Multiprocess directly. However, both of these options seem hacky.

I propose that running Uvicorn with --workers 1 runs the application in a subprocess using Multiprocess process manager. This is both a useful feature and seems more intuitive. Happy to open a PR.

This will also address the issue discussed here: #2390