CLN: de-kludge quantile, make interpolate_with_fill understand datetime64
2 related-but-separate things here

-quantile: de-kludging the base class method and putting the datetimetz-specific stuff in a subclass method.  This will allow us to implement the refactor suggested in #14562.  If/when DTA supports 2D, we'll be able to get rid of the subclass override kludge altogether.

- interpolate_2d: have the core.missing function understand datetime64 dtypes so the blocks don't have to implement special handling.  There are only a couple other places where this is needed before we can get rid of the datetimetz-specific try_coerce_result altogether.