Introduces a `WP_REST_Search_Controller` class which registers a `/wp/v2/search` endpoint. Search types are handled by extending `WP_REST_Search_Handler`. The default search type is `WP_REST_Post_Search_Handler` but can be filtered by plugins or a theme.
Merges [43739,43741] from the 5.0 branch to trunk.
Props danielbachhuber, flixos90, pento, rmccue, joehoyle.
Fixes#39965.
git-svn-id: https://develop.svn.wordpress.org/trunk@44107 602fd350-edb4-49c9-b593-d223f7449a82
Gary Pendergast