Track pull requests awaiting a response

A number of the issues or pull requests on our backlog are awaiting a response from the author (e.g. requires merge conflict resolution, requested changes on a review, or we have asked a question).

It would be good to follow on from #2627 and #2636 to track these issues with a label.

We could potentially use a workflow to automatically apply the label in some cases.

We should also consider whether we want to automatically close such issues after a certain time - however this requires some consideration, as in some cases we may be able to progress the issue/pull request without a response.

Read issues #2627 and #2636 first to understand the existing backlog-label approach. Define which unanswered cases should be tracked, whether automation is appropriate, and what policy should apply to closing them; the work is done when those decisions are agreed and the resulting tracking behavior is implemented.

Written by the indexing model from the issue text.