Define a ContactManager / ContactSchedule / ContactPhase interface

[wxmerkt]

1. Create a ContactManager msg that contains a set of ContactSchedule msgs, each contact schedule per each leg.
2. Redefine the ContactSchedule msg in such a way it contains a set of ContactPhase msgs, each one defines timings and /or foothold target. The first phase should be active, the second inactive, and so on. Inactive phases need to define foothold target.
3. Create the ContactPhase msg as defined in 2.

This format mimics exactly the definition of a contact diagram introduced in Caracal.
It works for any kind of robot.
Below, I provide the code for each message.

• ContactManager msg

std_mgs/Header
whole_body_mpc_msgs/ContactSchedule[] contacts

• ContactSchedule msg

string name
whole_body_mpc_msgs/ContactPhase[] phase

• ContactPhase msg

float64 duration
float64 step_height
geometry_msg/Pose target

Note that Caracal doesn't need the starting foothold, it uses the current one
But, we could publish the starting footstep placement if you think it is relevant for display purpose.

Originally posted by @cmastalli in #4 (review)