`add_target_date` isn't supported for multiple aheads

[dsweber2]

This is somewhat an issue for smooth_quantile_reg, which has multiple target dates/aheads. Specifically in layer_add_target_date, though likely we would also need to modify pivot_quantiles_wider to also pivot the target_date if needed.

Snippet that goes ~50% of the way to solving this when replacing the end of slather.layer_add_target_date

if (length(target_date)>1) {
  target_date = list(target_date)
}
object$target_date <- list(target_date)
components$predictions <- components$predictions %>%
  mutate(target_date = list(target_date))