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

David Faure requested to merge work/dfaure/modernize_kparts_usage into master

Test plan:

  • removed lib64/plugins/
  • 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:

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