Allow selecting OpenXR blend modes needed by see-through AR devices

Currently QtQuick3D.XR module is hardcoded to use XR_ENVIRONMENT_BLEND_MODE_OPAQUE blend mode (see XrEnvironmentBlendMode docs), but letting apps select e.g. XR_ENVIRONMENT_BLEND_MODE_ADDITIVE blend mode would add support for see-through headsets, where anything drawn black appears transparent. Now running Qt XR apps on any AR devices that don't do opaque (VR, pass-through AR) won't work.

In comparison Unity queries (and picks from) the available blend modes using OpenXR function xrEnumerateEnvironmentBlendModes() without requiring making changes to the application side code.