Allow expressions in `clin_group_by()` `group_by` argument

[mstackhouse]

Right now, groups are provided using variable names. Users may want to provide expressions instead. For example:

ct <- clintable(x) %>%
  clin_group_by(c(paste0("Laboratory Test (unit): ", param),
                               paste0("Treatment Group: ", tr01ag1)))

One approach for this could be adding a new parameter to provided external vectors. The internal logic of the function could then use the provided full length vectors instead of looking at ct$body$dataset:

ct <- clintable(x) %>%
  clin_group_by(group_by_vec = list(
    paste0("Laboratory Test (unit): ",x$param),
    paste0("Treatment Group: ",x$tr01ag1))
  )