Allow Zeroconf Spotify authentication to use multiple accounts and connect in the Spotify app in the local network

[flip-rossi]

This would allow anyone in my house to use our would-be-Mopidy sound system with their own Spotify account in a very plug-and-play manner, which is highly desirable and is what's keeping me from fully changing to Mopidy.

Because spotifyd uses this, it can be run without any user credentials, and instead authenticates with your account by making itself available in the Spotify app's "Select another device on this network" menu.

Zeroconf authentication on librespot's documentation: https://github.com/librespot-org/librespot/blob/master/docs/authentication.md#zeroconf-based-authentication

[flip-rossi]

This would also provide an alternate solution to issue #303's proposal

[flip-rossi]

Just noticed this is a duplicate of #14. Sorry

Still, shame it's not a feature yet :/

I unfortunately don't have the availability to try to take on something like this, but the last mention of librespot on that thread seems to be before this was a feature there as well. Maybe some inspiration can be taken by how spotifyd is doing it?