feat: add JointCommand.msg for semantic forward control

[ShahazadAbdulla]

Summary

This PR introduces JointCommand.msg to control_msgs. This provides a timestamped, semantically clear alternative to the deprecated std_msgs/Float64MultiArray currently utilized in forward_command_controller.

Justification & Context

1. Safety Timeouts: The current use of Float64MultiArray lacks a header stamp, which prevents the implementation of watchdog mechanisms. This new message includes a std_msgs/Header to unblock safety timeouts (referencing ros-controls/ros2_controllers#95).
2. Unified Controller Architecture: Following the deprecation of specialized forward controllers (ref: ros-controls/ros2_controllers#1913), the generic forward_command_controller requires a way to identify command types dynamically. The interface_name field explicitly defines whether a command is for position, velocity, effort, etc., without requiring a "message explosion" of unique types.
3. Resolves Deprecation Debt: This unblocks the cleanup of deprecated std_msgs and example_interfaces usage tracked in ros-controls/ros2_controllers#157 and gz_ros2_control#292.

Contribution Checklist

• Limited Scope: Only adds the new message definition and updates build files.
• Descriptive Title: Provided.
• Pipeline: Verified local build on ROS 2 Rolling and passed all pre-commit linters (ament_lint_cmake, ament_copyright, etc.).
• Testing: Verified message generation and member accessibility via ros2 interface show.

Message Structure

• std_msgs/Header header: Timestamp for watchdog/timeout logic.
• string[] joint_names: Explicit targeting of joints.
• string interface_name: Defines the interface (e.g., "position", "velocity").
• float64[] values: The commanded values.

Once merged, I will submit the corresponding PR in ros2_controllers to integrate this message and implement the safety watchdog logic.