Fix Sse client api (#6685)

* Use reqwest eventsource for get_events api

* await for Event::Open before returning stream

* fmt

* Merge branch 'unstable' into sse-client-fix

* Ignore lint

beacon_node/beacon_chain/tests/store_tests.rs

@@ -2796,6 +2796,7 @@ async fn finalizes_after_resuming_from_db() {
     );
 }
 
+#[allow(clippy::large_stack_frames)]
 #[tokio::test]
 async fn revert_minority_fork_on_resume() {
     let validator_count = 16;