Port assets_writer to ROS2

[chargerKong]

#47 Add assets_writer_node which can be called by

ros2 run cartographer_ros assets_writer_node \
-configuration_directory=xxx \
-configuration_basename=xxx.lua\
-bag_filenames=xxx\
-pose_graph_filename=xxx.pbstream\
-output_file_prefix=xxx\
-urdf_filename=xxx

where urdf_filename is optional

This code is tested in foxy, so I submit it to ros2 branch