The Patrick administration may require Amazon.com to start collecting sales tax on purchases made in Massachusetts, a move that would raise prices for online shoppers, but generate as much as $45 million in annual revenue for the state, according to several people briefed on the matter. A group of retailers, elected officials, and labor unions, recently escalated pressure on officials to impose the 6.25 percent tax on Amazon sales after the world’s largest Internet retailer bought a local technology firm and opened an office in Cambridge.