[FEATURE] Indexed Search: add mySQL fulltext support

Adds some minor modifications for restructuring in the pi code,
and some hooks in order to allow for other search engines to be
used.
Additionally, "indexed_search_mysql" is added to the core that
allows the mysql fulltext search with it.
Thanks to Michael Stucki for all the work!

Change-Id: I19ebc78dd0bb79297ee78104c7c6979379f2001e
Resolves: #28613