This feature request is already logged in our backlog so I marked this one as duplicated. You can cast your vote for the implementation as well as subscribe to the task by clicking the Follow button so you can receive updates when its status changes.
As a side note, I would like to mention that this functionality is partially developed. You can import/export documents with a text rotation set or you can set the text rotation using the CellSelection.SetTextRotation() method :
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