Diastereoselective Synthesis of Protected Vicinal Amino Alcohols: (S)‐2‐[(4S)‐N‐tert‐Butoxycarbonyl‐2,2‐Dimethyl‐1,3‐Oxazolidinyl]‐2‐tert‐Butyldimethylsiloxyethanal from a Serine‐Derived Aldehyde