How to build a `.so` dynamic lib on macOS, for building multi-platform jars

[ktoso]

We want to build a jar that contains libraries for various platforms, e.g. we COULD build a "fat jar" with both x86 and arm and .so and .dylib files etc.

We also want to cross build from macos to linux so develop on macOS and make a jar that works on linux.

In order to make .so files from macOS we'll need to use https://github.com/swiftlang/swift-sdk-generator/

I don't have a complete picture of how we'll do that but we'll need to figure it out.