GraphQL: Reposition epic within a list

While #233440 (closed) is about adding support to move an epic between lists this issue should add the possibility to move an epic within a list.

It could be similar to:

mutation {
  epicMoveList(input: { groupPath: "group", iid: "3", boardId: "gid://gitlab/Board/15", moveBeforeId: 100, moveAfterId: 99}) {
    errors
  }
}

We need to:

• create a service for reordering epics within a list
• add/modify the mutation

• Related MR: Adding relative positioning for boards
• Related issue: #247867 (closed) - decided we’ll have board-specific positions