`add_target_date` isn't supported for multiple aheads

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))

```