feature request: validator for number of reviews requested

[abid-mujtaba]

User Story: An author creates a PR but does not ask for a review (explicitly). I want a validator based on the number of reviews requested to trigger adding a "triage" label (when no reviewers have been added) indicating that this PR needs to be triaged and a reviewer assigned by the domain owner.

In addition, the "triage" label so applied will be used to add comments requesting a reviewer on a more frequent schedule.