Add example to demonstrate the use of QSGRenderNode with external 3D APIs in Qt6

Qt 5 had a nice example showing how to integrate QSGRenderNode with external 3D APIs such as Metal and D3D12. In Qt 6, the new example has been updated to show how one can use this class to issue draw commands with QRhi inline with the Qt Quick scenegraph. However, it'd be nice if the example could be extended to show how the same thing could be done with raw Vulkan, Direct3D11 and/or Metal APIs.