WheelSlipParametersCmd: add friction (#464)

This adds lateral and longitudinal friction parameters
to the WheelSlipParametersCmd message to match the
parameters supported in the gazebo_ros_wheel_slip plugin.

Signed-off-by: Steve Peters <scpeters@openrobotics.org>

proto/gz/msgs/wheel_slip_parameters_cmd.proto

@@ -69,4 +69,15 @@ message WheelSlipParametersCmd
  /// the linear wheel spin velocity and divided by the wheel normal force
  /// parameter specified in the sdf.
  double slip_compliance_longitudinal = 5;
+
+ /// \brief Lateral friction coefficient.
+ ///
+ /// Friction coefficient that should be applied with the pyramid friction
+ /// model in the lateral wheel direction.
+ double friction_lateral = 6;
+ /// \brief Longitudinal friction coefficient.
+ ///
+ /// Friction coefficient that should be applied with the pyramid friction
+ /// model in the longitudinal wheel direction.
+ double friction_longitudinal = 7;
 }