By BRIAN HANDWERK Ashwagandha is suddenly ev­erywhere. A growing interest in natural healthcare—plus the enthusiastic endorsements of celebrities and influencers—has led to a surge in curiosity about the me­dicinal plant. But there’s nothing new about ash­wagandha; it’s been used in tradi­tional Indian medicine, or Ayurveda, for thousands of years. Such staying power signals that many…