By William James and Farouq Suleiman LONDON (Reuters) -British former finance minister Rishi Sunak said on Friday he was running to replace Boris Johnson, three days after helping to launch the cascade of resignations that brought the prime minister down. Johnson announced on Thursday that he would stand down as prime minister after a mass rebellion in his Conservative Party, triggered by the latest in a series of scandals that had fatally undermined public trust. Johnson’s imminent departure has added political uncertainty to an already difficult mix of soaring inflation, slowing growth and i…