feat: Sleep Timer support

[drobotk]

Is your feature request related to a problem? Please describe. Spotify has a built-in sleep timer on its mobile app (iOS/Android) that fades music/podcasts out after a set time (5 min, 10, 15, 30, 45, 1 hr, or end of track/episode) by opening the Now Playing screen, tapping the three dots (or moon icon for podcasts), and selecting "Sleep timer". Librespot currently doesn't support the Sleep Timer feature. Trying to turn it on while connected to Librespot results in a popup saying Sleep Timer cannot be used on this device.

Describe the solution you'd like An implementation of the feature.

Motivation I often play some music on my speakers when going to bed. The problem is, I have to manually turn off the music, and if I fall asleep with it playing, I wake up to it still playing. The Sleep Timer feature is made exactly for this purpose and I'd like Librespot to support it.

Additional context