Build Unity project as part of CI pipeline

[randomPoison]

Currently only the Rust portion of the codebase is built as part of the CI pipeline. We should also build client code, including generating the shared Rust code that's used in the client. The unity-test-runner action should provide a reasonable way to get started. We'd only be able to run the Unity tests on Linux, but that would at least allow us to test standalone builds and potentially Android builds down the line.