[GQL] Add author and merged-at fields to MergeRequest type

What does this MR do?

Addresses #217475 (closed)

This adds missing features that prevent replacing uses of the REST API with GraphQL. These include:

• author field in MergeRequest type
• mergedAt field in MergeRequest type

Example GraphQL queries

query {
  project(fullPath: "gitlab-org/gitlab-test") {
    mergeRequests {
      nodes {
        title
        mergedAt
        author { username }
      }
    }
  }
}

Example GraphQL responses

{
  "data": {
    "project": {
      "mergeRequests": {
        "nodes": [
          {
            "title": "Cannot be automatically merged",
            "mergedAt": null,
            "author": {
              "username": "root"
            }
          },
          {
            "title": "Can be automatically merged",
            "mergedAt": null,
            "author": {
              "username": "root"
            }
          },
          ...
        ]
      }
    }
  }
}

Screenshots

Demonstrating use of new filters and fields

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.