feat: Multiagent functionality

[TannerGilbert]

This PR adds multiagent functionality to the Ardupilot simulation.

Previous discussions:

• Add multi-agent heterogenous demo with zephyr and iris in one environment ardupilot_gazebo#89
• https://discuss.ardupilot.org/t/multi-vehicle-zephyr-and-iris-ardupilot-gazebo-simulation-going-to-different-points-in-gazebo-simulation-for-the-same-lat-lon-alt/90686/4
• Add multi-agent launch file #65

This work builds on #64 extending the spawning via launch file functionality from just the Lidar example to all models available in this repository. The multiagent part is an extension of https://github.com/srmainwaring/ardupilot_gz/tree/prs/pr-multi-vehicle. It takes into account the proper namespace handling for all nodes including RVIZ where the config was switched to use relativ paths and the ros_gz_bridge where the robot name is dynamically swapped out.

Example usage:

ros2 launch ardupilot_gz_bringup multiagent.launch.py

Further changes:

• Extracted common code from iris_lidar.launch.py, iris.launch.py and wildthumper.launch.py into robot.launch.py to decrease duplicated code
• Added gui launch argument to make it possible to not start Gazebo GUI

[srmainwaring]

Thanks @TannerGilbert, this looks like a great addition. I'll try to get this running under my macOS / Jazzy setup and give feedback.

[TannerGilbert]

@srmainwaring Thanks. If you have any feedback or desired improvements please let me know. Just FYI I developed + tested this on Ubuntu 22.04 with ROS 2 Humble.