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