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>

components/net/protocols/mod.rs

@@ -14,6 +14,7 @@ use log::error;
 use net_traits::filemanager_thread::RelativePos;
 use net_traits::request::Request;
 use net_traits::response::Response;
+use servo_url::ServoUrl;
 
 use crate::fetch::methods::{DoneChannel, FetchContext, RangeRequestBounds};
 
@@ -47,6 +48,15 @@ pub trait ProtocolHandler: Send + Sync {
     fn is_fetchable(&self) -> bool {
         false
     }
+
+    /// Specify if this custom protocol can be used in a [secure context]
+    ///
+    /// Note: this only works for bypassing mixed content checks right now
+    ///
+    /// [secure context]: https://developer.mozilla.org/en-US/docs/Web/Security/Secure_Contexts
+    fn is_secure(&self) -> bool {
+        false
+    }
 }
 
 #[derive(Default)]
@@ -114,9 +124,22 @@ impl ProtocolRegistry {
     pub fn is_fetchable(&self, scheme: &str) -> bool {
         self.handlers
             .get(scheme)
-            .map(|handler| handler.is_fetchable())
-            .unwrap_or(false)
+            .is_some_and(|handler| handler.is_fetchable())
     }
+
+    pub fn is_secure(&self, scheme: &str) -> bool {
+        self.handlers
+            .get(scheme)
+            .is_some_and(|handler| handler.is_secure())
+    }
+}
+
+/// Test if the URL is potentially trustworthy or the custom protocol is registered as secure
+pub fn is_url_potentially_trustworthy(
+    protocol_registry: &ProtocolRegistry,
+    url: &ServoUrl,
+) -> bool {
+    url.is_potentially_trustworthy() || protocol_registry.is_secure(url.scheme())
 }
 
 pub fn range_not_satisfiable_error(response: &mut Response) {