[Feature Request]: merge_srv(output_name, join_fun) to be reactive

### Feature description

Reactive should be allowed for these arguments. `tm_cross_table` `have input$join_fun` which allows to change merge function during app-run. (select x and y from different datasets)

```r
library(teal.modules.general)
options(
shiny.useragg = FALSE,
teal.ggplot2_args = teal.widgets::ggplot2_args(labs = list(caption = "NEST PROJECT"))
)

## Data reproducible code ----
data <- teal_data()
data <- within(data, {
library(random.cdisc.data)
ADSL <- radsl(seed = 1)
ADRS <- radrs(ADSL, seed = 1)
})

join_keys(data) <- default_cdisc_join_keys[c("ADSL", "ADRS")]

app <- init(
data = data,
modules = modules(
tm_t_crosstable("Table Choices")
)
)

shinyApp(app$ui, app$server)
```