First PR for#2.

This PR adds an abstraction for downloading & validating the dylib from a Coder server, and the network extension scaffolding.

It also adds aTunnelHandle type for owning the pair of pipes passed to the dylib, and the handle to the dylib itself.

You cannot create a unit test target that targets a System Extension. So, this PR extracts the portion of the network extension that we'd like to test into it's own Framework,VPNLib.

Of note is thatSwiftProtobuf doesn't have a stable ABI (as it doesn't uselibrary evolution), so the Framework has theBuild libraries for distribution setting disabled. This shouldn't effect anything. Exporting theSwiftProtobuf types should be fine provided we don't importSwiftProtobuf in to theVPN target as well.