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.
When authoring the bundle, you specify a registry location where the path the exe will be.  So at runtime, the engine gets the exe path.