feat(event-stream): Support block_gossip topic (#15038)

* feat(event-stream): Add block_gossip topic support

* feat(event-stream): Add block_gossip topic support

* feat(event-stream): Add block_gossip topic support

* feat: add block gossip topic support to beacon api event stream

* fix: sync_fuzz_test panic

* fix: check for nil operationNotifier before sending block gossip

The operationNotifier was not being checked for nil before being used,
which could lead to a panic if it was not initialized. This commit adds
a nil check to prevent the panic.

---------

Co-authored-by: james-prysm <90280386+james-prysm@users.noreply.github.com>

api/server/structs/endpoints_events.go

@@ -20,6 +20,11 @@ type BlockEvent struct {
 	ExecutionOptimistic bool   `json:"execution_optimistic"`
 }
 
+type BlockGossipEvent struct {
+	Slot  string `json:"slot"`
+	Block string `json:"block"`
+}
+
 type AggregatedAttEventSource struct {
 	Aggregate *Attestation `json:"aggregate"`
 }