add follow_heading for FOLLOW decision

modules/planning/optimizer/dp_st_speed/dp_st_graph.cc

@@ -430,6 +430,7 @@ bool DpStGraph::CreateFollowDecision(
   follow_point->set_x(path_point.x());
   follow_point->set_y(path_point.y());
   follow_point->set_z(0.0);
+  follow->set_follow_heading(path_point.theta());
 
   return true;
 }

modules/planning/proto/decision.proto

@@ -56,6 +56,7 @@ message ObjectYield {
 message ObjectFollow {
   optional double distance_s = 1;  // minimum longitutional distance in meters
   optional apollo.common.PointENU follow_point = 2;
+  optional double follow_heading = 3;
 }
 
 message ObjectOvertake {