Add optional add_mtc launch argument to enable MTC ExecuteTaskSolution capability

[gadorneles]

MoveIt Task Constructor (MTC) requires the ExecuteTaskSolution capability to be loaded in move_group for task.execute() to work. Without it, MTC can plan tasks but execution fails because the /execute_task_solution action server is not advertised.

This is added as an opt-in launch argument rather than enabled by default because it requires the ros-${ROS_DISTRO}-moveit-task-constructor-capabilities package to be installed. Enabling it unconditionally would break move_group startup for users who don't have that package. I've only added it upstream to xarm6_moveit_fake.launch.py because that's where I've tested it, but I can add it to the others if necessary.

Summary

• Adds an add_mtc launch argument (default false) to _robot_moveit_common.launch.py, _robot_moveit_common2.launch.py, _robot_moveit_fake.launch.py, and xarm6_moveit_fake.launch.py
• When add_mtc:=true, the move_group/ExecuteTaskSolutionCapability capability plugin is loaded into move_group, enabling the /execute_task_solution action server required by MoveIt Task Constructor

Usage

ros2 launch xarm_moveit_config xarm6_moveit_fake.launch.py add_gripper:=true add_mtc:=true

Prerequisite

  sudo apt install ros-${ROS_DISTRO}-moveit-task-constructor-capabilities

Test plan

• Launch with add_mtc:=false (default) — verify move_group starts normally, /execute_task_solution is not in ros2 action list
• Launch with add_mtc:=true — verify /execute_task_solution appears in ros2 action list
• Launch with add_mtc:=true without the capabilities package installed — verify move_group reports a clear plugin loading erro