[WIP] Add PS4 controller interface in ExternalFootstepPlannerPlugin

[TsuruMasato]

Hello, Dr.Arnaud.
This is Tsuru. This PR is a kind of progress report.

To integrate your large development of LIPM walking controller with AvatarController, I am now working on connection with Joystick.

========== Motivation ==============
I have read a new, your FSM state in AvatarController, "ROSInputState", it was very easy to integrate ROS inputs at once.

The shortest way is assigning some mc_datastore variables from ROSInputState.
But in future, we might want to use LIPM walking controller alone, without Avatar Controller.
So that I, embedded a Joystick Subscriber in LIPM walking controller today.

========== How I made ==============
Originally, there were 3 options, "World SE2", "Local SE2", and "Local Velocity".
So I have added 4th option, "PS4 Controller".

A new ROS subscriber locates in ExternalFootstepPlannerPlugin, and arise when users select "PS4 Walking" mode in a pull-down via RViz control panel.

My callback function receives sensor_msgs::Joy and assigns a new local target (SE2d) immediately.
Therefore, same as before, what any controller have to do is only calling the datastore service "ExternalFootstepPlanner::RequestPlan".

========== Future work ==============
Also, we may develop a new footstep planner and replace OnlineFootstepPlanner.cpp with it in future.
Even that case, we can easily use this joystick input in this PR, without using AvatarController.

About AvatarController, I might use your ROSInputState in the end.
But even at that case, I will call my new datastore service of "ExternalFootstepPlanner::SetJoystickVelocityTarget".
This function may be extended as input_convertor_node, which remaps buttons & keys from several kinds of controllers (Occulus and PS4).

[TsuruMasato]

I have implemented Input Convertor class in the ExFootstepPlannerPlugin.