WASHINGTON — America’s employers added a solid 223,000 jobs in December, evidence that the economy remains healthy even as the Federal Reserve is rapidly raising interest rates to try to slow economic growth and the pace of hiring. All told, the December jobs report suggested that the labor market may be cooling in a way that could aid the Fed’s fight against high inflation. What’s more, average hourly pay growth eased in December to its slowest pace in 16 months.