Synthesis, Antimicrobial and Antitubercular Activity of Some Novel [3‐Isonicotinoyl‐5‐(4‐substituted)‐2,3‐dihydro‐1,3,4‐oxadiazol‐2‐yl] and Substituted 5‐(Pyridin‐4‐yl)‐1,3,4‐oxadiazole‐2‐thiol Derivatives.