[TIMOB-17111] iOS: APSHTTPClient should not wrap NSURLResponse

APSHTTPClient should not wrap NSURLResponse. Apple's docs, videos and examples consistently demonstrate and recommend that a url response should be immutable, and that readonly properties should be added to NSURLRequest via subclassing. Additionally, the url response should only include the metadata of the server's response and not the actual data - these two concepts are independent and should be modeled independently (they are currently clubbed together).