What a way to make your red carpet relationship debut! Ben Foster and Laura Prepon hit the The Girl On The Train premiere Tuesday night, marking their first official public appearance as a couple … as well as the debut of a new diamond ring on a very tell-tale finger. Speculation was at an all-time high as she flashed the new sparkler — and as it turns out, this one is the real deal, sources confirm to PEOPLE. Laura Prepon and Ben Foster Are Engaged! See Her Gorgeous New Ring If this is the first time you’ve even heard about these two dating, you’re not alone. It was only back…