Pull in the required MonoPoxisHelper method calls into this repo.

Currently for .net 6 and other platforms we have a runtime dependency on Mono.Posix.NetStandard. This is only because we need libMonoPosixHelper.so/dylib on the various platforms. We already pulled in the required C# code so we are independent of that library. So we should also pull in the required native bits.

Now we can either

1. Create a new native lib `libZipSharpHelper` which contains those calls or..
2. Fold that code into the `libzip` native libraries.

Not sure which option is better..