Add noteable commenters GraphQL field

What does this MR do and why?

We currently have to enumerate all notes on merge requests for reporting everyone who contributed (e.g. everyone who commented). This is hugely inefficient: gitlab-org/quality/toolbox!108 (merged)

This MR introduces a commenters field which simply returns the distinct list of note authors (pushing the processing to the server and hugely reducing the data we need to fetch).

Screenshots or screen recordings

How to set up and validate locally

    query {
      projects {
       nodes {
         fullPath
         issues {
          nodes {
            id
            iid
            commenters {
              nodes {
                username
                name
              }
            }
          }
        }
       }
     }
   }

MR acceptance checklist

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