Some of the nation’s wealthiest people, including a handful from Massachusetts, are asking the federal government to raise their taxes under the “Buffett Rule.’’ Because the current tax code imposes only a 15 percent levy on capital gains, many affluent Americans pay lower effective tax rates than low- and middle-income earners. “It’s incumbent upon people like me, who’ve been successful, to pay it forward so the next kid has the same opportunity I had,’’ said entrepreneur Paul Egerman.