XR: Improve OpenXR instance management
This MR adds two features:
- A new static function,
vtkOpenXRManager::QueryInstanceVersion
, that returns the OpenXR runtime version for given connection strategy. This function enables programs to get this version without instantiating a render window. - Small modification in
vtkOpenXRManagerConnection::Initialize
to reset the environment variable set byvtkOpenXRRemotingManagerConnection::Initialize
. This enable dynamic switch between OpenXR remoting runtime and default OpenXR runtime.