Add support for qt_helper_lib()

Add qt_add_3rdparty_library() function as a replacement for qmake's
qt_helper_lib feature.

All 3rdparty libraries will be available under the Qt:: alias when built
through this method so that they can properly register as dependencies
of a Qt module.

This patch also adds Qt3rdPartyLibraryConfig.cmake.in to export the
CMake configuration for static builds and shared libraries.

Change-Id: I52bf3a95ca22fccd9ab54343468847bb1b570c28
Fixes: QTBUG-81969
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>