Split NIOHTTPServer into separate module

[aryan-25]

Motivation:

Both the abstract HTTP server API and the concrete NIO implementation are currently contained in the same module. We should separate the concrete NIO implementation into its own module.

Modifications:

Added a new NIOHTTPServer target and updated import statements. The resulting structure is:

Sources/HTTPServer
├── HTTPRequestContext.swift
├── HTTPResponseSender.swift
├── HTTPServer.swift
├── HTTPServerClosureRequestHandler.swift
├── HTTPServerRequestHandler.swift
└── RequestResponseMiddlewareBox.swift
Sources/NIOHTTPServer
├── HTTPRequestConcludingAsyncReader.swift
├── HTTPResponseConcludingAsyncWriter.swift
├── NIOHTTPServer.swift
├── NIOHTTPServer+ConnectionContext.swift
├── NIOHTTPServer+HTTP1_1.swift
├── NIOHTTPServer+ListeningAddress.swift
├── NIOHTTPServer+SecureUpgrade.swift
├── NIOHTTPServer+SwiftConfiguration.swift
├── NIOHTTPServerConfiguration.swift
├── NIOSSL+X509.swift
└── SocketAddress.swift

Result:

The abstract HTTP server API and the concrete NIO implementation are now split in different modules.

[tib]

LGTM