Support drag-and-drop loading of bags

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)

rerun-io / cpp-example-ros2-bridge