NEW YORK — The Brooklyn Nets gave Spencer Dinwiddie a contract extension Thursday, a day after the best game of his career. The three-year extension will be worth about $34 million for the guard who was playing in the NBA G League before he signed with the Nets on Dec. 8, 2016. He has gone on to become one of their top players and scored a career-high 39 points, the most by any player off the bench in the NBA this season, in a victory at Philadelphia on Wednesday. “The journey is just beginning,” Dinwiddie wrote on Twitter.