* prepare to migrate st_boundary to path_obstacle.

Planning: allow empty st_graph_boundary; do not return error when no mapping of obstacle. abstract redundant code to seperate function.

refactored the MapStopDecision function in StGraphMapper

and decisions

main decision can be abstracted into a stop obstacle
Mission Complete should be processed in high level code, such as frame
or planner

* it is not an Impl of a pure interface. Impl is misused here
* The class is more like a collection of functions, which can be
improved.

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

* easier for coding
* less error prone for building ST graph.

added st_boundary_mapper.h/cc; warning appear at build because hdmap hasn't support for LaneInfo.is_on_lane function.