Provide access to OpenXR instance and session handles and state changes

Currently, QtQuick.XR module provides a subset of the functionality exposed by standard OpenXR 1.0 and 1.1 specification, and no way to access optional OpenXR extensions, which are often essential for implementing productised experiences on specific VR/XR hardware.

Calling most OpenXR APIs and extensions requires passing references to the app's OpenXR instance or session, which are hidden behind private bits of QQuick3DXrManager. Unity for example provides OpenXR Features extension mechanism with OpenXRFeature API with functions like OnInstanceCreate(ulong xrInstance), OnSessionBegin(ulong xrSession), OnSessionStateChange(int oldState, int newState), etc. that allow creating custom OpenXR functionality and extending Unity OpenXR support with new XR features.