Fix eth2 compilation by feature gating SseEventSource (#8819)

`eth2` is currently unable to be built without the `events` feature.


  Feature gates the `SseEventSource` match arm in the `status` function.


Co-Authored-By: Mac L <mjladson@pm.me>

common/eth2/src/error.rs

@@ -102,6 +102,7 @@ impl Error {
                     None
                 }
             }
+            #[cfg(feature = "events")]
             Error::SseEventSource(_) => None,
             Error::ServerMessage(msg) => StatusCode::try_from(msg.code).ok(),
             Error::ServerIndexedMessage(msg) => StatusCode::try_from(msg.code).ok(),