[wix-devs] Bal.wixext BA x64/ARM64 support

[Sean Hall]

How do we want to do this? In the extension custom actions, there's a
custom element like NativeImage in NetFx. Its compiler calls
ParseHelper.CreateCustomActionReference which picks the right Id based on
the architecture being compiled.

For the BAs, there is no custom element. The user would manually have to
set the BootstrapperApplicationRef Id based on which architecture they're
compiling for.

Do we want to create a new element to abstract this away? Should we
repurpose the
WixStandardBootstrapperApplication/WixManagedBootstrapperApplicationHost
elements for this? It seems like the Bal extension was originally put
together with the goal to write as little C# code as possible.