This PR aims to add BLS12-381 native library (#13), taken from this implementation.

Things to consider:

• BLS12 library supports linux, mac & windows (x64).
• I've simplified the build process, making it more maintainable.
• Added JNA for jni bindings (same as besu's)
• Added osxcross: macOS Cross toolchain for Linux and *BSD
• Productive build only runs linux test cases -> tested mac and win manually (when it's possible)

NOTE: to facilitate this review you can filter the first added bls12-381 library commit, which only has a copy paste of bls library.