pr-auditor: add support for 'no review required'

Created by: bobheadxi

You can now indicate a No review required: blah blah anywhere in your test plan to indicate that your PR does not need a review ahead of time. This will mark your PR as reviewed.

See thread:

now that this process is in place for the managed instance repo, we’re forced to get a review on normal upgrade procedures. We don’t normally ask for reviews on these, they’re boilerplate operations. It’s not a huge deal to change our process, but I’m not sure it’s adding any value because the changes are already live.

This will likely also be needed to enable workflows where bots make image updates that should be automatically deployed.

Note that this will not bypass repo-permissions-level branch protection requiring review, but it allows repositories to implement workflows that don't require reviews while still maintaining an audit log.

Test plan

unit tests for days