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.
What about other directories like `%ProgramData%`? Perhaps an array of allowed directory roots could be used instead, and then you can avoid the duplicate code and just loop through until you've found (or not) the EXE under a secure, allowed root.