Restrict MQTT JSON downlink messages (#3141)

Channel needs to be named "mqtt"
"from" field should be set to the node number of the transmitter

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mqtt/MQTT.h

@@ -5,6 +5,7 @@
 #include "concurrency/OSThread.h"
 #include "mesh/Channels.h"
 #include "mesh/generated/meshtastic/mqtt.pb.h"
+#include "mqtt/JSON.h"
 #if HAS_WIFI
 #include <WiFiClient.h>
 #define HAS_NETWORKING 1
@@ -100,6 +101,9 @@ class MQTT : private concurrency::OSThread
     void publishStatus();
     void publishQueuedMessages();
 
+    // returns true if this is a valid JSON envelope which we accept on downlink
+    bool isValidJsonEnvelope(JSONObject &json);
+
     /// Return 0 if sleep is okay, veto sleep if we are connected to pubsub server
     // int preflightSleepCb(void *unused = NULL) { return pubSub.connected() ? 1 : 0; }
 };