By Jonathan Spicer and Michelle Nichols ISTANBUL/UNITED NATIONS (Reuters) – Ukraine, Russia, Turkey and U.N. Secretary-General Antonio Guterres will sign a deal on Friday to resume Ukraine’s Black Sea grain exports, Turkish President Tayyip Erdogan’s office said on Thursday. Russia and Ukraine are both major global wheat suppliers, but Moscow’s Feb. 24 invasion of its neighbor has sent food prices soaring and stoked an international food crisis. The war has stalled Kyiv’s exports, leaving dozens of ships stranded and some 20 million tonnes of grain stuck in silos at Odesa port. Ankara said a g…