add PipeWire audio backend and mixer

[gearhead]

This PR adds a pipewire native backend to spotifyd. It also allows spotify to use the normal wireplumber mixer (stream) or can also use the pipewire sink mixer if desired.

The config needs to have these settings is pipewire is used:

backend = "pipewire"
volume-control = "pipewire"

if you want the spotifyd volume to control the pipewire 'sink', add this:
pipewire_mixer_mode = "sink"

PipeWire stream volume control is assumed if this is not present. This means there are 2 volume controls in series, the pipewire stream then the actual PipeWire sink volume. This is desirable in some settings, whereas if this is running on a dedicated player, the sink volume may be more desirable.

This has been tested on a RPi running 64 bit RPiOS.