Add a "supported" property to SpatialAnchors and Hands QML types.

Smilar to how we have https://doc.qt.io/qt-6/qml-qtquick3d-xr-xrview.html#passthroughSupported-prop and how it is used in our examples to set or not set passthrough depending on its availability, we could add this to other APIs that some platforms lack.