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.
Good point. We should have functions that do this already since I know we do it for our "standard" CAs. You'd have to poke around a bit, though. Maybe we already do it in Burn anyway for MSI property command lines.