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::*;