audioconvert: add zeroramp and gap detection to audioconvert

Make a new zeroramp.duration and zeroramp.gap property on audioconver.
It detects N samples of silence before triggering a fade-in or fade-out
of the given zeroramp.duration.

The zeroramp.duration is by default 5ms and zeroramp.gap is set to 0.

When the zeroramp.gap is set to 0, the audioconver will not do any gap
detection but it will only do fade-out from the last sample when the IO
Buffer area is removed from a port.

This by default makes the audio adapter perform a fade-out when the last
input of the port mixer was removed and the mixer is no longer scheduled
and the IO Area removed from the audioconverter input port.

doc/dox/config/pipewire-client.conf.5.md

@@ -87,6 +87,7 @@ stream.properties = {
     #dither.noise = 0
     #dither.method = none # rectangular, triangular, triangular-hf, wannamaker3, shaped5
     #debug.wav-path = ""
+    #zeroramp.gap = 0
     #zeroramp.duration = 0.005
 }
 ```