Planning: use two step qp st search process. use smaller acceleration range in the first step, if not possible, extend the acceleration range and search again.

Planning: changes for follow algorithm. (1) make sure acceleration is < 0 when the adc is between the front car and follow fence; (2) only add follow reference point when the point is behind reference line; (3) added parameter follow_drag_distance to adjust following behavior.

* prepare to migrate st_boundary to path_obstacle.

bugfix: fix reference guide line bug
add unit test

Planning: revert changes of speed_limit because t info is needed. fixed some bugs in qp_spline_st_graph.

* not all points defined in path_point.proto are path points.
* the points defined in path_point.proto are commonly used in PnC modules