arrow

ARROW-13200: [R] Add binding for case_when()

Adds support for `case_when()` in dplyr verbs. I followed the example of `dbplyr::case_when()`, which is much simpler and more self-contained than `dplyr::case_when()`.

Closes #10737 from ianmcook/ARROW-13200

Lead-authored-by: Ian Cook <ianmcook@gmail.com>
Co-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Ian Cook <ianmcook@gmail.com>