WIXFEAT:3249 - Allow BA to run elevated async process through the engine.

Add ApprovedExeForElevation element, IBootstrapperEngine::LaunchApprovedExe, IBootstrapperApplication::OnLaunchApprovedExeBegin, IBootstrapperApplication::OnLaunchApprovedExeCompleted, and enable WixStdBA to use them.
Maybe I missed it or haven't gotten to it yet, but what's the registry access for? Seems passing the ApprovedExe ID to the elevated process should be enough. I don't remember seeing you marshal anything else across.