Add macOS Support for Testing in GitHub Actions

[nijikon]

Currently, the test suite for the rdkafka-ruby library does not include execution on macOS runners. As macOS is a commonly used platform among developers, especially in the Ruby ecosystem, ensuring that the library works seamlessly on macOS is essential.

GitHub Actions provides hosted runners for macOS (macos-latest), which can be integrated into the existing CI pipeline to test the library on this platform automatically.