[ENH] first/last/all matches for conditional join (#1114)

* convert extension arrays to numpy arrays for _range_indices

* add keep parameter

* use list comprehension for some cases of first/last keep

* keep equality conditions logic simple and foolproof

* changelog

* docs cleanup

Author: samukweku

CHANGELOG.md

@@ -3,7 +3,7 @@
 ## [Unreleased]
 
 -   [DOC] Updated developer guide docs.
--   [ENH] Allow column selection/renaming within conditional_join. #1102 @samukweku.
+-   [ENH] Allow column selection/renaming within conditional_join. #1102. Also allow first or last match. #1020 @samukweku.
 -   [ENH] New decorator `deprecated_kwargs` for breaking API. #1103 @Zeroto521
 -   [ENH] Extend select_columns to support non-string columns. Also allow selection on MultiIndex columns via level parameter. #1105 @samukweku
 -   [ENH] Performance improvement for groupby_topk. #1093 @samukweku