Stereocontrolled Preparation of 3‐Acyltetrahydrofurans from Acid‐Promoted Rearrangements of Allylic Ketals: (2S,3S)‐3‐Acetyl‐8‐Carboethoxy‐2,3‐Dimethyl‐1‐Oxa‐8‐Azaspiro[4.5]Decane