Find better way of having opaque std::unique_ptr<T> in an exported class ?

[rouault]

This is related to https://github.com/OSGeo/gdal/pull/10762

Having something like the following in a public header

class CPL_DLL Foo
{
   private:
          class Bar;
          std::unique_ptr<Bar> opaque{};
}

prevents such header from being used by external code with Visual Studio