Tells the client that the URL Loading System received an authentication challenge.
- iOS 2.0+
- macOS 10.2+
- tvOS 9.0+
- watchOS 2.0+
The URL protocol object sending the message.
The authentication challenge that has been received.
The protocol client guarantees that it will answer the request on the same thread that called this method. The client may add a default credential to the challenge it issues to the connection delegate, if
protocol did not provide one.