fix(types): add sentTime to FirebaseMessagingTypes.RemoteMessage (#3885)

[publish]

packages/messaging/lib/index.d.ts

@@ -97,6 +97,11 @@ export namespace FirebaseMessagingTypes {
      */
     ttl?: number;
 
+    /**
+     * The time the message was sent, in milliseconds since the start of unix epoch
+     */
+    sentTime?: number;
+
     /**
      * Any additional data sent with the message.
      */