Support drag-and-drop loading of bags

[roym899]

It would be nice to add an external data loader for ROS bags to allow drag-and-drop viewing.

To get started, see C++ example.

Some notes:

• there is no nice way to configure this loader from the viewer, maybe a config file from the drag-and-dropped dir or from the CWD could be used?
• for ROS 1 a bag is a single file
• for ROS 2 up to and including Humble a bag is a directory
• for ROS 2 starting from Iron it can be either a file or a directory
• should be automatically added to the path when the ROS environment is sourced (similar to rviz)