feat: add SubscriptionHandle for managing subscription lifecycle

- Add SubscriptionHandle struct to manage async subscription tasks
- Implement graceful shutdown mechanism with unsub_fn callback
- Export new module in common/mod.rs
- Support for proper resource cleanup in subscription management

src/common/mod.rs

@@ -3,5 +3,6 @@ pub mod nonce_cache;
 pub mod tip_cache;
 pub mod types;
 pub mod address_lookup_cache;
+pub mod subscription_handle;
 
 pub use types::*;

src/common/subscription_handle.rs

@@ -0,0 +1,13 @@
+use tokio::task::JoinHandle;
+
+pub struct SubscriptionHandle {
+    pub task: JoinHandle<()>,
+    pub unsub_fn: Box<dyn Fn() + Send>,
+}
+
+impl SubscriptionHandle {
+    pub async fn shutdown(self) {
+        (self.unsub_fn)();
+        self.task.abort();
+    }
+}