marker: forward data_link_type on per-link marker create

The per-link create_marker route (links.py) called start_marker without the data_link_type from the body, so it always defaulted to DLT_EN10MB and a serial encapsulation chosen by the caller was silently dropped. Same oversight the IOU capture and definition routes had; one-arg fix.

gns3server/api/routes/controller/links.py

@@ -469,6 +469,7 @@ async def create_marker(
         capture_node_id=marker_data.capture_node_id,
         color=marker_data.color,
         highlight_duration=marker_data.highlight_duration,
+        data_link_type=marker_data.data_link_type,
     )
     return link.markers.get(name, {})