Remove dependency on Foundation

I have been looking into removing the dependency on `Foundation` from [`soto`](https://github.com/soto-project/soto/compare/main...t089:soto:essentials) and [`soto-core`](https://github.com/soto-project/soto-core/compare/main...t089:soto-core:essentials). I got really far without too much trouble, but then realized that this library has a pretty strong dependency on `Foundation` that is not easily resolvable as many of the types used here are not available in `FoundationEssentials`. Such as `NSNumber`, `NSNull`, `JSONSerialization`.

Not sure what the best approach would be, maybe it would be better to instead use a different JSON library such as [IkigaJSON](https://github.com/orlandos-nl/IkigaJSON).