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>

components/constellation/tracing.rs

@@ -209,6 +209,7 @@ mod from_script {
                 Self::MoveTo(..) => target_variant!("MoveTo"),
                 Self::ResizeTo(..) => target_variant!("ResizeTo"),
                 Self::Prompt(..) => target_variant!("Prompt"),
+                Self::RequestAuthentication(..) => target_variant!("RequestAuthentication"),
                 Self::ShowContextMenu(..) => target_variant!("ShowContextMenu"),
                 Self::AllowNavigationRequest(..) => target_variant!("AllowNavigationRequest"),
                 Self::AllowOpeningWebView(..) => target_variant!("AllowOpeningWebView"),