[Utils] Add ci_results script

Adds a script that can help inspecting and root cause CI results. We
provide two commands, `current-status` and `failure-range`. The first
summarizes the results from the most recent run on all CI bots, and the
second helps bisect where a particular test started failing.

This all works by using the `gh` tool to look at the logs from github
workflow runs, with some smarts that have to do with how our runs are
set up.

Author: bogner