Add support for osx-arm64 runtimes

[tvandinther]

First raised in #17, the FFI binaries are only built for x64 on Linux, Windows and MacOS. MacOS running on Apple silicon is common enough to require support.

Expected changes:

A new build step targetting a Github Actions runner using Apple silicon to build an additional FFI binary.

Github has recently released into beta their macos-14 runner which uses the M1 chip. A new build step can be created targetting this runner.
https://docs.github.com/en/actions/using-github-hosted-runners/about-github-hosted-runners/about-github-hosted-runners#standard-github-hosted-runners-for-public-repositories

Copying the new osx-arm64 binary into the package

The csproj file will need to be changed to distinguish between and copy the x64 and arm64 binaries into their respective runtime directories. According to the MSBuild docs this should be runtimes/osx-arm64/native