The hottest amenity in New York City’s luxury rental market is making a big splash this summer. Developers and landlords across the five boroughs are crowning their projects with bigger and better outdoor pools. Long a staple in high-end city living, it’s now must-have feature for high-end apartment seekers who are increasingly willing to sign a lease — or walk away from one — based solely on whether their building comes with a place to swim in the sky. The latest entrant making the biggest