gst: factor out the stream management and some common variables in a new class

Construct this new class from both the src and sink to be able to share code

Consolidate the previous mess of open/close/start/stop into a single pair
of open/close functions in the new stream class

src/gst/meson.build

@@ -6,6 +6,7 @@ pipewire_gst_sources = [
   'gstpipewirepool.c',
   'gstpipewiresink.c',
   'gstpipewiresrc.c',
+  'gstpipewirestream.c',
 ]
 
 if get_option('gstreamer-device-provider').allowed()
@@ -20,6 +21,7 @@ pipewire_gst_headers = [
   'gstpipewirepool.h',
   'gstpipewiresink.h',
   'gstpipewiresrc.h',
+  'gstpipewirestream.h',
 ]
 
 pipewire_gst = shared_library('gstpipewire',