Storing xgboost models in data.tables fails

Hey, we try to make mlr3 compatible with the new xgboost R package 3.1.2.1. We store models in `data.table` list columns. This does not work anymore with the new version.

```r
library(data.table)
library(xgboost)

data(mtcars)

model_regression = xgboost(mtcars[, -1], mtcars$mpg, nthreads = 1, nrounds = 3)

data.table(model = list(model_regression))

# Error in y[] <- lapply(y, reallocate) :
# ALTLIST classes must provide a Set_elt method [class: XGBAltrepPointerClass, pkg: xgboost]
```

Start by running the reported R reproduction with xgboost 3.1.2.1 and data.table, then inspect how the model's XGBAltrepPointerClass is stored in a list column. Done means data.table(model = list(model_regression)) succeeds without the ALTLIST Set_elt error and preserves the model.

Written by the indexing model from the issue text.