Add support for Sane Airscan

[dvkch]

https://github.com/alexpevzner/sane-airscan

The work has been started to compile the library on macOS, but it requires Avahi and DBus to work. We need to do a port of the library that would use NetServiceBrowser instead of Avahi, and replace the DBus runloop by NSRunLoop on macOS/iOS.

This is a huge undertaking and I will not have the time to work on it just yet, but will keep it in mind for future free times :)

Once the work is done and upstreamed, it should only be a matter of integrating it into Sane-build.rb and Sane.xcframework.