Move SocketAddress type from HTTP Server repo into this (or some other) repo

[gjcairo]

We are currently using a SocketAddress (i.e. IP address) type in the NIO implementation (swift-server/swift-http-server#32) but we should move this type to some shared repo instead as it should be a shared-with-the-client / currency type.

See swift-server/swift-http-server#32 (comment)