Add Support for Swift's AsyncHTTPClient

Created by: tanner0101

AsyncHTTPClient is a new HTTP client library for Swift built on SwiftNIO. This library utilizes non-blocking IO which makes it ideal for use Swift servers.

From what I can tell OpenAPI Generator currently supports generating Swift 5 clients based on URLSession, Alamofire, and RxSwift. It would be great if we could add support for AsyncHTTPClient so that servers could generate non-blocking clients to interact with OpenAPI-compatible APIs.