systemd: configure pipewire/wireplumber/pipwire-pulse user daemons for socket activation

it looks like pipewire can be socket activated, so we just need to be sure the service/socket files are packaged in the systemd-services pkg and installed as user services, and test that they work OOTB.

Note: this originally was written to include pulseaudio, but after careful consideration I (@craftyguy) have decided that it's not worth having systemd manage pulseaudio. It's configured to autospawn, and there are a lot of UI elements in GNOME, etc that will cause it to start. This mechanism seems to work well enough, it's restarted on crash, etc.