Move smoltcp-related code to bigtcp

kernel/libs/aster-bigtcp/src/socket/event.rs

@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: MPL-2.0
+
+/// A observer that will be invoked whenever events occur on the socket.
+pub trait SocketEventObserver: Send + Sync {
+    /// Notifies that events occurred on the socket.
+    fn on_events(&self);
+}
+
+impl SocketEventObserver for () {
+    fn on_events(&self) {}
+}