Add model_reference attribute to RoadMarking (#874)

Signed-off-by: Andreas Rauschert <andreas.rb.rauschert@bmw.de>

Author: arauschert

osi_roadmarking.proto

@@ -96,6 +96,13 @@ message RoadMarking
     //
     optional ColorDescription color_description = 5;
 
+    // Opaque reference of an associated 3D model of the road marking.
+    //
+    // \note It is implementation-specific how model_references are resolved to
+    // 3d models.
+    //
+    optional string model_reference = 6;
+
     //
     // \brief \c Classification data for a road surface marking.
     //