Port the lunar pole exploration rover plugins and model to Gazebo Harmonic (keeping Fortress)

[asimonov]

The lunar pole exploration rover is the one model in this repository with custom Gazebo system plugins — SolarPanelPlugin, RechargeableBatteryPlugin, SensorPowerSystemPlugin and RadioisotopeThermalGeneratorPlugin — and they only build for Ignition Fortress. Since #41 they are excluded from non-Humble builds, so on ROS 2 Jazzy the simulation package builds with no plugins and the rover has no power system. The rover's .gazebo xacro is also Ignition-only (ign_ros2_control/IgnitionSystem, ignition-gazebo-*-system plugin names), and its ros2_control configuration lives in the demos repository, referenced through $(find lunar_pole_exploration_rover).

This blocks moving the lunar_pole_exploration_rover demo off the Humble base image (space-ros/demos#164).

Proposal

• Port the four plugins to the gz-sim 8 API (gz::sim, gz/ headers, GZ_ADD_PLUGIN, per-message includes for gz-msgs 10) while keeping them building on Fortress through a small compatibility header, since Fortress ships the gz/ redirect headers but not the GZ_-prefixed macros. Select the Ignition system packages on Humble and the ROS gz_*_vendor packages otherwise, so the plugins link against the same libraries as ros_gz and gz_ros2_control.
• Make the rover model select ign_ros2_control / ignition-gazebo-* or gz_ros2_control / gz-sim-* names from ROS_DISTRO, and keep its controller configuration next to the model, as the curiosity rover does.
• Set the GZ_SIM_* resource and plugin paths in the package's environment hook alongside the IGN_GAZEBO_* ones, so Harmonic finds the meshes and the plugins (this is the hook part of Update to Gazebo Harmonic/Jazzy #36).

Humble stays fully supported; several open PRs still target it.