net: clean shutdown of the async runtime (#38425)

The previous use of a static variable for the runtime prevented it from
shutting down cleanly, because shutdown requires dropping or taking
ownership of it. This PR switches the static variable to a handle only,
and introduces a new trait to pass a handle to the async runtime to the
constellation, where it can be shut-down along with other components and
help reduce our count of still running threads after shutdown.

Testing: manual testing, and covered by unit-test in net, and wpt tests.
Fixes: part of - https://github.com/servo/servo/issues/30849

---------

Signed-off-by: gterzian <2792687+gterzian@users.noreply.github.com>

components/shared/net/lib.rs

@@ -389,6 +389,12 @@ impl<T: FetchResponseListener> Action<T> for FetchResponseMsg {
     }
 }
 
+/// Handle to an async runtime,
+/// only used to shut it down for now.
+pub trait AsyncRuntime: Send {
+    fn shutdown(&mut self);
+}
+
 /// Handle to a resource thread
 pub type CoreResourceThread = IpcSender<CoreResourceMsg>;