Expose single merge request within a project GraphQL query

What does this MR do?

This exposes a single MergeRequest (can be reused for others) within a project GraphQL query:

query($project_path: ID!) {
  project(fullPath: $project_path) {
    id
    mergeRequest(iid: "1") {
      title
    }
  }
}

To return

{
  "data": {
    "project": {
      "id": "3",
      "mergeRequest": {
        "title": "Soluta vitae deserunt omnis quia quis magni sed voluptatem assumenda tenetur."
      }
    },
  }
}

This came up while we were discussing https://gitlab.com/gitlab-org/gitlab-ce/issues/47695, where we were using this style of query in the examples, and I liked it, but we don't support it yet. This adds those, reusing the existing resolvers.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/47393