gst: add keepalive-time property

Add a property to periodically send the last buffer to keep the
stream alive. Useful for sparse streams that need to keep the
encoder busy every once and a while.

src/gst/gstpipewiresrc.h

@@ -66,6 +66,7 @@ struct _GstPipeWireSrc {
   gint max_buffers;
   int fd;
   gboolean resend_last;
+  gint keepalive_time;
 
   gboolean negotiated;
   gboolean flushing;