Support binding to unix domain sockets

It would be nice if `shiny run` would expose a cli option to bind to unix domain sockets instead of tcp ports.
`uvicorn` already supports this via the `--uds` flag.

This already works in python (as `run_app()` passes `**kwargs` on to `uvicorn.run()`):

```
from shiny import run_app

run_app("app:app", uds="/tmp/shiny.sock")
```

But in the `shiny run` cli, additional arguments are not allowed.