Make Librespot Credentials Optional

[airdrummingfool]

librespot does not require credentials in order to work.

from the README:

librespot can be run in discovery mode, in which case no password is required at startup. For that, simply omit the --username argument.

This PR allows both username and password to be omitted. If one is provided and the other is not, an error is thrown.

[badaix]

What is the discovery mode? Can it play audio in this mode? What is the difference to the "Spotify connect" mode (my understanding is that in the "Spotify connect" mode, it can be used as a remote player instance).
I currently don't have a payed account, so I cannot test it.

[airdrummingfool]

I've always used discovery mode in my setup, because I don't like entering my username and password. From what I can tell, "Discovery Mode" starts librespot in a state that is waiting for an auth token, which it gets from another client when you select the librespot instance in a client's Spotify Connect list.

Discovery Mode can play audio. I don't think it's actually different than "Spotify Connect" mode, except that it has to wait to get auth info from another Spotify client (which happens automatically when you tell librespot to play music from another client). It still functions as a remote player.

[badaix]

aha, cool 👍