Ament index ressources through unified cli interface

Hi,

While jumping from project to project you might want to know what resources are available for discoverability (plugins, config files or other custom resources).
And ament_index_python provide the exact tooling in order to do that. However this is yet another endpoint one user needs to know.
What do we think about having integration with ros2cli. And proposing a new package named ros2index doing exactly what ament_index_python proposes.