The secret’s out! On April 26, Naomi Watts was spotted walking her dog outside the Greenwich Hotel in NYC wearing a stunning diamond ring on that finger, sparking speculation that boy-friend Billy Crudup popped the question. “They’ve been keeping the engagement hush-hush,” says Life & Style’s source. “Naomi’s only told her closest friends and family, but of course she said yes.” It will be the first marriage for both Billy and Naomi. The actor, 54, previously dated Claire Danes and Mary-Louise Parker, mom to his 19-year-old son Will, while the actress, 54, was with Liev Schreiber for 11 years without ever tying the knot. (The exes share two children, Sasha, 15, and Kai, 14.) “Neither Naomi nor Billy has been married before, but it’s something they’d talked about,” dishes the…