libservo: Add a delegate method for HTTP authentication (#35400)

Add a delegate method for HTTP authentication and a related
`AuthenticationRequest` object that carries with it the URL as well as
whether or not the authentication request is for a proxy or not.

This is now separate from the prompt API because requesting
authentication doesn't necessarily involve prompting -- this is an
implementation detail of the embedder. In addition, the internal bits
are cleaned up slightly.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

ports/servoshell/egl/app_state.rs

@@ -255,10 +255,6 @@ impl WebViewDelegate for RunningAppState {
             PromptDefinition::Input(message, default, response_sender) => {
                 response_sender.send(cb.prompt_input(message, default, trusted))
             },
-            PromptDefinition::Credentials(response_sender) => {
-                warn!("implement credentials prompt for OpenHarmony OS and Android");
-                response_sender.send(Default::default())
-            },
         };
     }