New branch and fix API breaking to allow Humble to work with Gazebo Garden

[federicociresola]

Hi guys, I created this PR to allow Humble to work with Gazebo Garden. I started to work with iron branch because it contains the porting from ignition to gazebo API. I discovered that the ros2_control repository has changed the API from Humble to Iron. Therefore, I updated the code with old version of ros2_control's API.

It seems to work without problem with Humble and Garden. My idea is to keep a two different branches, so a person can choose if need to work with Gazebo Garden or Ignition Fortress.

Tell me what do you think about this idea