Add GraphQL query to fetch single epic board list

What does this MR do and why?

Related to #355947 (closed)

Adds the ability to find an epic board list by gid. This is needed to fetch list metadata like totalWeight as a deferred query.

Example query

query {
  epicBoardList(id: "gid://gitlab/Boards::EpicList/3", epicFilters: {}) {
    id
    metadata {
      totalWeight
    }
  }
}

Screenshots or screen recordings

How to set up and validate locally

MR acceptance checklist

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