It’s a diamond in the ruff . Cash-flush canine lovers can bestow their pups with ultra-luxe,18-karat gold and diamond custom collars — including one for a whopping $150,000 — thanks to a collaboration between the Baccarat Hotel New York and The D Diamond , a self-proclaimed luxury jeweler for canine companions. “There’s different tiers within jewelry. I would qualify this as fine jewelry,” one D Diamond client, Monica Bates, told The Post. Every dog apparently does have its day — especially