nixos/pulseaudio: make daemon.conf configurable (#20888)

This adds pulseaudio.daemon.config, which is a set of keys to values
which are directly translated to keys and values of pulseaudio's
daemon.conf, e. g.

    hardware.pulseaudio.daemon.config = { flat-volumes = "no"; }

becomes

    flat-volumes=no

in pulse/daemon.conf.