Revert "Revert "Add WaitForSynced to beacon node for validator startup (

#5366)""

This reverts commit 927d8d9.

beacon-chain/core/feed/state/events.go

@@ -9,6 +9,8 @@ const (
 	ChainStarted
 	// Initialized is sent when the internal beacon node's state is ready to be accessed.
 	Initialized
+	// Synced is sent when the beacon node has completed syncing and is ready to participate in the network.
+	Synced
 )
 
 // BlockProcessedData is the data sent with BlockProcessed events.
@@ -27,6 +29,12 @@ type ChainStartedData struct {
 	StartTime time.Time
 }
 
+// SyncedData is the data sent with Synced events.
+type SyncedData struct {
+	// StartTime is the time at which the chain started.
+	StartTime time.Time
+}
+
 // InitializedData is the data sent with Initialized events.
 type InitializedData struct {
 	// StartTime is the time at which the chain started.