Initial support for marking custom protocol secure (#36656)

Add initial support for marking custom protocol as secure, this makes it
possible to `fetch` a custom protocol inside secure contexts (e.g.
http://localhost)

Some additional contexts:

- [#embedding > Custom protocol secure
context](https://servo.zulipchat.com/#narrow/channel/437943-embedding/topic/Custom.20protocol.20secure.20context)
-
https://github.com/versotile-org/tauri-runtime-verso/issues/6#issuecomment-2820776128

Testing: use `fetch('urlinfo://abc').then(async (response) =>
console.log(await response.text())).catch(console.log)` in servoshell
with in a secure context (e.g. https://servo.org), and see the response
should not be an error

---------

Signed-off-by: Tony <legendmastertony@gmail.com>
Signed-off-by: Tony <68118705+Legend-Master@users.noreply.github.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

ports/servoshell/desktop/protocols/urlinfo.rs

@@ -46,4 +46,8 @@ impl ProtocolHandler for UrlInfoProtocolHander {
     fn is_fetchable(&self) -> bool {
         true
     }
+
+    fn is_secure(&self) -> bool {
+        true
+    }
 }