kigpart: add JSON metadata to plugin, install into kf5/parts subdir

Test plan:
 - removed lib64/plugins/kigpart.so
 - `kig` still works (finds the part on startup)
 - konqueror in filter/tests still allows to preview kig and kgeo files
 - lxr shows no other code loading "kigpart" by name directly

KF6 task: https://phabricator.kde.org/T12340