GraphQL - Expose epics under board list

New GraphQL endpoint to allow fetching epics in board list

All epics from all lists:

query {
  group(fullPath: "group") {
    board(id: "gid://gitlab/Board/3") {
      name
      lists {
        node {
          id
          epics {
            nodes {
              title
            }
          }
        }
      }
    }
  }
}

Epics from one list:

query {
  group(fullPath: "group") {
    board(id: "gid://gitlab/Board/3") {
      name
      list(id: "gid://gitlab/List/8") {
        id
        label {
          title
          color
        }
        epics {
          nodes {
            title
          }
        }
      }
    }
  }
}

Tasks

• Add a new table to store epic boards (e.g. epic_boards) (#233436 (comment 425401673))
• Create new GraphQL endpoint to expose Boards
• Create new GraphQL endpoint to expose Lists in a Board
• Create new GraphQL endpoint to expose Epics in a Board List
  • Make sure that relevant indexes (relative_position) are created when listing epics