[Feature Request]: `values(keep_na = logical(1))` to handle `NA`

### Feature description

Related to very old issue present in des also: https://github.com/insightsengineering/teal.transform/issues/30

We already found a quite nice solution in [`teal_slice(keep_na)`](https://insightsengineering.github.io/teal.slice/latest-tag/reference/teal_slice.html). In `teal.slice` when column contains any `NA`, checkbox input is automatically added to the filter-state-card. We expect similar in `pick_ui`

Consider following example

```r
data <- within(teal.data::teal_data(), {
iris <- iris
mtcars <- mtcars
iris$Species[1:5] <- NA
})

app <- init(
data = data,
modules = modules(
modules(
label = "Testing modules",
tm_merge(
label = "non adam",
picks = list(
a = picks(
datasets("iris", "iris"),
variables(
choices = c("Sepal.Length", "Species"),
selected = "Species"
),
values()
)
)
)
)
)
)

shinyApp(app$ui, app$server, enableBookmarking = "server")

```

1. how NA should be included in choices/selected?

```r
values(choices, selected, keep_na = TRUE/FALSE)
# adds keep_na to the attributes of the `pick` object
```

2. how NA should be displayed handled in the `pick_ui/srv`?

- checkbox-input should be added below pickerInput (or rangeSlider or other input related with class of the column)
- changing input should update attribute in the object

3. how NA should be handled in merge_srv

Reactive `keep_na` should be delivered somehow to `merge_srv > .qenv_merge > .merge_expr > .call_dplyr_filter` and handled respectively.