Swift implementation of NAT type detection

Inspired by https://github.com/HMBSbige/NatTypeTester
Issue and pull request are welcomed

Swift Package Manager

The Swift Package Manager is a tool for automating the distribution of Swift code and is integrated into the swift compiler.

Once you have your Swift package set up, adding SwiftNATTypeDetector as a dependency is as easy as adding it to the dependencies value of your Package.swift.

dependencies: [
    .package(url: "https://github.com/swarm-cloud/SwiftNATTypeDetector.git", .upToNextMajor(from: "0.0.1"))
]

NAT类型探测的Swift实现

参照了c#版的实现：https://github.com/HMBSbige/NatTypeTester
欢迎测试效果并反馈issue