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

[gogonzo]

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)

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)