enhance: Expose a listen interface

https://github.com/fosrl/newt/blob/b6f13a1b5531e26e530f14e0d2b172c399a8deaa/authdaemon/server.go#L24-L33

Currently we only expose a port option but setups where you dont want to bind to all interfaces it would be useful to allow `listen` or the base of the `addr` for a specific interface.

then https://github.com/fosrl/newt/blob/b6f13a1b5531e26e530f14e0d2b172c399a8deaa/authdaemon/server.go#L126

can be something like

```go
s.addr = fmt.Sprintf("%s:%d", cfg.Listen, cfg.Port)
```

if listen is empty string then we still get the bind to all interfaces `:port`

**brain dump** maybe `bind` is better terminology.