Make the UDP socket's .with pub

This helps develop embassy-rs#2516
out-of-tree.

Not cherry-picked from any PR.

embassy-net/src/udp.rs

@@ -87,6 +87,7 @@ impl<'a> UdpSocket<'a> {
         }
     }
 
+    /// Lock the stack, provide access to the underlying smoltcp UDP socket and interface
     fn with<R>(&self, f: impl FnOnce(&udp::Socket, &Interface) -> R) -> R {
         self.stack.with(|i| {
             let socket = i.sockets.get::<udp::Socket>(self.handle);