KF6CoreAddonsMacros: Allow to use targets in kcoreaddons_target_static_plugins

This way, one can import plugins from other KDE frameworks.
In addition to just the lib, we need to specify the target name and its
plugin namespace as properties. These also need to be exported.

Because kcoreaddons_target_static_plugins can take either a namespace or
list of targets, it is cleaner and more extendible to use arg parsing
instead of a positional parameter.