[C++][Compute] Improve top_k/bottom_k Selecters via CRTP

As it was mentioned here: 

https://github.com/apache/arrow/pull/11019#discussion_r701349253

Selecters for SelectKUnstable all have a relatively similar core structure. It might be worth considering how some templating (e.g. via CRTP, or via a set of helper comparator/iteration templates) could let you factor out the core algorithm and the container-specific bits.

It would then be easier to also try to share generated code between types with the same physical type (e.g. as mentioned, Int64, Timestamp, and Date64 should all use the same generated code underneath). This issue  related to  create template specialization functions for these types  was also mentioned here: 

https://github.com/apache/arrow/pull/11019#discussion_r700238908