arrow

ARROW-14052: [C++] Add approximate_median aggregation

This wraps the tdigest aggregation to make it easier for bindings to use. This PR is on top of ARROW-14050 (#11199).

Closes #11204 from lidavidm/arrow-14052

Authored-by: David Li <li.davidm96@gmail.com>
Signed-off-by: David Li <li.davidm96@gmail.com>