Add createdAt to Runner GraphQL API

What does this MR do and why?

Part of #347856 (closed)

Add createdAt to Runner GraphQL API

Why this change?

Our designs at require displaying a creation time for our runners: #347856 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. Visit the GraphQL explorer http://gdk.test:3000/-/graphql-explorer
2. Use this example GraphQL query:

fragment RunnerNode on CiRunner {
  id
  description
  createdAt
}

query getRunners($before: String, $after: String, $first: Int, $last: Int, $status: CiRunnerStatus, $type: CiRunnerType, $tagList: [String!], $search: String, $sort: CiRunnerSort) {
  runners(before: $before, after: $after, first: $first, last: $last, status: $status, type: $type, tagList: $tagList, search: $search, sort: $sort) {
    nodes {
      ...RunnerNode
    }
  }
}

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #347856 (closed)