Convert choices_selected to data_extract_spec with only filter_spec

Usage

cs_to_des_filter(
  cs,
  dataname,
  multiple = FALSE,
  include_vars = FALSE,
  label = "Filter by"
)

cs: (choices_selected) object to be transformed. See teal.transform::choices_selected() for details.
dataname: (character) name of the data
multiple: (logical) Whether multiple values shall be allowed in the shiny selectInput.
include_vars: (flag)
whether to include the filter variables as fixed selection in the result. This can be useful for preserving for reuse in rtables code e.g.
label: (character) Label to print over the selection field. For no label, set to NULL.