Add output type to mqtt node

Fixes #1912

nodes/core/locales/zh-CN/messages.json

@@ -314,6 +314,7 @@
         "label": {
             "broker": "服务端",
             "example": "e.g. localhost",
+            "output": "输出",
             "qos": "QoS",
             "clientid": "客户端ID",
             "port": "端口",
@@ -342,6 +343,10 @@
             "connect-failed": "与服务端 __broker__ 的连接失败"
         },
         "retain": "保留",
+        "output": {
+            "buffer": "Buffer",
+            "string": "字符串"
+        },
         "true": "是",
         "false": "否",
         "tip": "提示: 若希望通过msg属性对topic(信息), qos及retain(保留)进行设置, 则将上述项留白",