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

Test plan:
- removed lib64/plugins/kmplotpart.so
- kmplot still works (finds the part on startup)
- konqueror still allows to preview a fkt file
- lxr shows no other code loading "kmplotpart" by name directly

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