[Feature Request] Support live statistics in progress bar

[tw-aisi]

Right now the progress bar that shows when one runs inspect eval does not show any running statistics. Sometimes a small fraction of evals can take twice as long or longer than the rest of evals, and it would be convenient in these scenarios to be able to view a running statistic to know how well the run is doing.

[jjallaire]

Note that we very soon be working on improved live task display (including both statistics as well as drilling down into what's happening w/ individual samples)