DDS: Vehicle to report SITL state

Nobody has claimed this yet.

ROS visualization and integration with other sensors require the vehicle's actual state rather than the estimated one, especially if we are using cameras or lidars as a measurement for estimating the state itself.

## Feature request

A new publisher `/ap/sim/groundtruth` with a custom message type, published at at least 50Hz in SITL only.

The message could be like:
```
std_msgs/Header header

# Geographic position
float64 latitude # degrees
float64 longitude # degrees
float64 altitude # meters above mean sea level

# Cartesian position in ENU frame relative to home position
geometry_msgs/Vector3 position

# Orientation quaternion in ENU frame
geometry_msgs/Quaternion orientation

# Linear velocity in ENU frame (m/s)
geometry_msgs/Vector3 linear_velocity

# Angular velocity in body frame (rad/s)
geometry_msgs/Vector3 angular_velocity
```

**Platform**
[ x ] All
[ ] AntennaTracker
[ ] Copter
[ ] Plane
[ ] Rover
[ ] Submarine

Start by tracing the existing ROS visualization and SITL integration entry points in ArduPilot. Define the custom message and publish it on /ap/sim/groundtruth only in SITL at least 50 Hz, covering the geographic, ENU position and orientation, and velocity fields listed in the issue. Done means the publisher reports the vehicle’s actual state for ROS consumers without affecting non-SITL platforms.

Written by the indexing model from the issue text.