Add position_in_list for epicMoveList mutation

What does this MR do and why?

Issue: #377862 (closed)

This adds the position_in_list param to the epicMoveList mutation. It's 0-indexed and by using -1 as value, an epic gets moved to the bottom of the list.

Screenshots or screen recordings

mutation

How to set up and validate locally

mutation moveEpic {
  epicMoveList(
    input: {
      boardId: "gid://gitlab/Boards::EpicBoard/1",
      epicId: "gid://gitlab/Epic/2810", 
      toListId: "gid://gitlab/Boards::EpicList/1", 
      positionInList: 0 # change to -1 to move it to the bottom of the list
    }
  ) {
    clientMutationId
  }
}

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