sg: add an audit command to display prs trail
Created by: jhchabran
This is a first iteration of https://github.com/sourcegraph/sourcegraph/issues/31725 that simply outputs a report of the issues in either markdown or text format of the issues created by missing test plans or reviews.
The plan is to automate it as we refine the process in the upcoming weeks.
I avoided documenting it on purpose, as I'm not sure we're going to keep this command when we'll roll out an automated version.
I'm looking in particular for feedback regarding the message being posted: we want it to be actionable and clear for the mentioned teammates.
I'll be posting the first message manually, and we can see how it'll go and decide on how we want to automate this, if we do.
Test plan
- checked the output in my terminal and in slack: https://sourcegraph.slack.com/archives/C02L31KTHHT/p1646762315082699