Utility to do ard_* calls over multiple filtered datasets

[bzkrouse]

With our stacking functions, we can easily get summaries for mutually exclusive subgroups + overall.

However, a gap exists for more complex subgroup combinations, like non-mutually exclusive subgroups or filtered portions of datasets. I think it would be a nice addition to have an ard mapper function that iterates over different subgroup filters or flags.

#522 would allow us to perform ard_* operations on a filtered chunk of a dataset, making it a key building block to this feature.

[ddsjoberg]

We would use the captured expression and the name in the labelling/registration of the data frame

list("named filter" = AGE > 50, "another name" = AGE > 30) |>
  foo()