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