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.
I'm open to more directories.  `%ProgramData%` isn't locked down to only allow administrators write access, so that's not a good example.