With C# records - which are available since C# 9, so covers Unity requirements as well - we can use subclassing and pattern matching to get sum types that look a lot more like Rust tagged enums.
This is a breaking change but IMO worth it for the better API going forward.
This allows to make and test ABI-breaking changes much more easily than before.
For now moved them under crates/bindings-csharp - on one hand, it's not a crate, but on another it's the most logical place within our current project structure if we think of `crates` folder as general `projects` folder.