Expose public CMake api to create a Qt plugin

Currently all plugins are built with qt_add_plugin, which is private CMake API.
qmake allows building a plugin by using something like the following in the .pro file

TARGET = qsqlite
QT  = core core-private sql-private
PLUGIN_TYPE = sqldrivers
PLUGIN_CLASS_NAME = QSQLiteDriverPlugin
load(qt_plugin)

We should expose a public version of qt_add_plugin probably via Qt6CoreMacros to allow building Qt plugins.

The question is what should the private counter part be called: qt_add_plugin would be the natural choice for the public api, so we need to rename the private one to something else?