Feature suggestion: Delegation support

[collares]

One big feature supported by third-party merge queues (such as Bors) is delegation, meaning granting someone else the permission to merge a particular PR (or add it to the merge queue) if status checks are OK, even if they're not a committer. This covers the use case where a reviewer says "I am happy with this PR except for a few nits; please do these changes, but there's no need for another round of reviews". It would be great to have native support for this in GitHub too, even if it comes with a big warning message about this being unsafe if you don't trust the PR author.