Add an option to make bucket notifications synchronous (#17406)

With the current asynchronous behaviour in sending notification events
to the targets, we can't provide guaranteed delivery as the systems
might go for restarts.

For such event-driven use-cases, we can provide an option to enable
synchronous events where the APIs wait until the event is successfully
sent or persisted.

This commit adds 'MINIO_API_SYNC_EVENTS' env which when set to 'on'
will enable sending/persisting events to targets synchronously.

internal/config/api/help.go

@@ -104,5 +104,11 @@ var (
 			Optional:    true,
 			Type:        "boolean",
 		},
+		config.HelpKV{
+			Key:         apiSyncEvents,
+			Description: "set to enable synchronous bucket notifications" + defaultHelpPostfix(apiSyncEvents),
+			Optional:    true,
+			Type:        "boolean",
+		},
 	}
 )