Add reviewers to PR

[matttrach]

Need the ability to get and set requested reviewers on PRs similar to labels.

{
  repository(owner: "k3s-io", name: "k3s") {
    id
    pullRequest(number: 7426) {
      id
      reviewRequests(first: 10) {
        totalCount
        nodes {
          requestedReviewer {
            ... on Team {
              combinedSlug
            }
            ... on User {
              email
            }
          }
        }
      }
    }
  }
}

Review requests is an ephemeral value, it disappears once someone adds a review. Requested Reviewer does not make it into the auto-generated functions.