Open source the Unity runtime native libraries

Despite Rive moving to a more open source model, the rive-unity package cannot be meaningfully contributed to as the underlying native libraries for this package are not open source. The github page states

The rive-unity runtime uses the latest Rive C++ runtime.

However, the exported symbols from the native libraries in the repository do not exist anywhere in any of the public rive repositories.

The biggest issue currently is that there is A LOT of missing functionaily regarding querying information in the unity package, which makes using and debugging issues very difficult currently. Being able to contribute at least to these more "grunt" level Get* functionality would vastly improve the current package.

This is following on from my support thread regarding here so this can be tracked and allow for public contributation. https://rive.app/community/forums/support/fsnLLkXUaA62/rive-unity-libraries-not-producable-from-rive-cpp/ftvXGDy5mpZj