Use DSV environment hook to register plugin

[StefanFabian]

The current setup procedure could be simplified by just exporting an environment hook that will register the plugin automatically for gazebo when the workspace is sourced.

Unfortunately, the documentation for that is really hard to find. I couldn't even find it myself, knowing that it existed at some point.
Here's some information I could find: gazebosim/ros_gz_project_template#16

See example dsv:
https://github.com/StefanFabian/qml_ros2_plugin/blob/master/environment_setup.dsv.in

and how to register:
https://github.com/StefanFabian/qml_ros2_plugin/blob/master/CMakeLists.txt#L170