"[A]s a practical matter, the fiduciary standards applicable to the offering of guaranteed lifetime withdrawal benefit (GLWB) products are not materially different than the standards applied to fiduciaries in connection with the selection of any other investment or guaranteed lifetime income option for an individual account plan.... This article is ... a roadmap of much of the available guidance, which fiduciaries can use to answer most of the questions that would reasonably be expected to arise if GLWBs were added to a plan's menu of investment options."