Bayer AG, one of the last big European pharmaceutical companies without a presence in the Boston area, may soon be setting up a local outpost to work with biotech startups and academic researchers on drug discovery. Bayer CEO Marijn Dekkers said in an interview Monday that the German company is in "advanced stages of discussion" about opening a site here modeled on its collaboration offices in San Francisco, Toyko, and Shanghai, which each employ up to 100 people.