gst: deviceprodiver: Add fd property / camera portal support

Add a `fd` property to `GstPipeWireDeviceProvider` so it can be used
with fds obtained from e.g. the camera xdg-desktop-portal.

This is needed so apps like Cheese or Camera can easily implement the
camera portal.

src/gst/gstpipewiredeviceprovider.h

@@ -58,6 +58,7 @@ struct _GstPipeWireDevice {
   GstPipeWireDeviceType  type;
   uint32_t            id;
   uint64_t            serial;
+  int                 fd;
   const gchar        *element;
 };
 
@@ -82,6 +83,7 @@ struct _GstPipeWireDeviceProvider {
   GstDeviceProvider         parent;
 
   gchar *client_name;
+  int fd;
 
   GstPipeWireCore *core;
   struct spa_hook core_listener;