testing/spotifyd: Run as openRC service

Ey Thomas, here is the MR that I told you by email.

It's pretty difficult to give a nice default configuration for this.

Do you think it should provide a default 'spotifyd.conf'? Where? /etc/spotifyd.conf or /etc/spotifyd/spotifyd.conf? I think that alsa is a good default backend. Useful for some embedded devices that someone want to user as music player, but on laptops/desktops with pulseaudio/pipewire it seems difficult to provide a working solution. In my case I'm using pipewire-pulse with a custom server on '/tmp/spotifyd' which I 'chmod 777' (so spotifyd runing as different user can send audio).

Ideally pulseaudio/pipewire should run as a system service but is not very common configuration. The better seems just leaving alsa as default and let the user do some magic if he wants.

I splitted it in openrc subpackage so people who doesn't want this can totally ignore it (maybe another point for don't provide a default /etc/spotifyd.conf)

Also it creates a new user/group spotifyd, which runs the service.