[TASK] Implement event handler trait for substrate

[salman01zp]

Implement the EventHandler trait similar to Evm here which will allow one event watcher to run and many event handlers that will run in parallel.

let vanchor_watcher_task = event_watcher.run(
      client,
      store,
      vec![
            Box::new(deposit_handler),
            Box::new(leaves_handler),
            Box::new(encrypted_output_handler),
          ],
       &my_ctx,
       );