CMake: cross-platform Webots controller build (macOS prebuilt vs Linux source)

[idesign0]

Summary

This PR improves the CMake build of webots_ros2_driver to support both Linux and macOS environments:

• Detect prebuilt /Applications/Webots.app on macOS and link against its libraries.
• Add support for source builds on macOS (similar to Linux).
• Use file(GLOB …) for Webots libraries instead of hardcoded list.
• Refactor Python version detection (humble/iron → 3.10, others → 3.12).
• Switch from yaml-cpp to yaml_cpp_vendor for consistency with ROS 2.
• Install Python packages only if directories exist.
• Conditional add_dependencies only when not using prebuilt Webots.
• webots-controller script installation now uses ${WEBOTS_CONTROLLER_EXEC}.

Motivation

These changes allow webots_ros2_driver to build on:

• Linux with Webots source build
• macOS with prebuilt Webots.app

Notes

@traversaro — I would greatly appreciate your guidance and feedback here. :)

Testing

• ✅ macOS (Apple Clang) with Webots.app 2025a (prebuilt)
• ✅ Ubuntu 22.04 with source Webots