Add support for Gazebo Ionic (gz-sim9)

[traversaro]

Gazebo Ionic (i.e. gz-sim9) is in the process of being released: https://community.gazebosim.org/t/community-meeting-ionic-september-2024/3038 .

We should add support for it, ideally keeping support also for Harmonic, at least for some time.

Due to the fact that for each gz-sim version the conda package names, cmake packages names are different, this may be a bit challenging:

• For what regards ci, I guess we can try to add support in Ionic first in pixi, by creating two different environments, one for ionic and one for harmonic. I did a prototype in https://github.com/robotology/gz-sim-yarp-plugins/tree/pixisupportionic .
• For what regards CMake, there are no helpers to just do find_package(gz-sim) and just find what is available in the system, see Convenience CMake for Gazebo Metapackage gazebosim/gz-cmake#69 . So probably we need to first try to find gz-sim9, and if it is not found try to find gz-sim8, perpahps saving the preference in a variable so that users may want to override directly it.