indexerconfig: Expose canBeSearched function

Summary:
Make D10205 public

CCBUG:388656

Reviewers: Baloo, Dolphin, dfaure, lbeltrame

Reviewed By: lbeltrame

Subscribers: lbeltrame, adridg, Frameworks

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D10661