Add `createdBy` to runner type

Description

Using the user-based runner creation workflow #383139 (closed) we now have information on who created a runner.

This information should be added to the GraphQL API:

query getRunner($id: CiRunnerID!) {
  runner(id: $id) {
    id
    createdBy ### UserCore (https://docs.gitlab.com/ee/api/graphql/reference/#usercore)
  }
}

Note: Other APIs use slightly similar names for this field such as createdBy or author, I use createdByUser as a placeholder.