Use single package for both iOS and macOS. Add support for SPM on macOS.

[Moriquendi]

1. Is there any reason why there are two separate repositories for macOS and iOS version of this library?
   It'd be way more convenient to have a single package for both platforms which conditionally compiles some source files.

2. macOS package doesn't support Swift Package Manager right now. Would be great if Agora could add support for it :)

[maxxfrazer]

Sorry I haven't been ontop of the issues on this repo!

1. Yes I completely agree; this is something being worked on, in addition to other apple platforms
2. I believe the sdk team are no longer supporting a separate audio SDK for macOS, and possibly iOS soon too. The size of the SDK is pretty much the same for audio vs video, so there's not much point to it.