The Reagan administration, in its last economic forecast, issued an optimistic outlook Tuesday predicting that the economy will grow at a robust 3.5 percent annual rate next year. The prediction for growth in the gross national product, which measures the total output of goods and services, is 1.3 percentage points higher than the consensus view of many private economists. Many private analysts are looking for growth to be very sluggish next year, with the GNP expanding at a rate of 2.2 percent.