Support all additive functions with rollup

As of today, only `sum`, `max` and `min` are supported in the `RealtimeToOfflineSegmentsTask`. We should be able to support all additive functions during rollup (eg. `percentile` etc)

Start by reading RealtimeToOfflineSegmentsTask and the existing handling for sum, max, and min. Trace how additive functions are represented and selected during rollup, then identify the relevant coverage for functions such as percentile. Done means all supported additive functions can be used during rollup rather than only the currently listed functions.

Written by the indexing model from the issue text.