Two years ago, Dr. Ugur Sahin took the stage at a conference in Berlin and made a bold prediction. Speaking to a roomful of infectious disease experts, he said his company might be able to use its so-called messenger RNA technology to rapidly develop a vaccine in the event of a global pandemic.   At the time, Dr. Sahin and his company, BioNTech, were little known outside the small world of European biotechnology start-ups. BioNTech, which Dr. Sahin founded with his wife, Dr. Özlem Türe