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/mesh/Channels.h

@@ -32,7 +32,7 @@ class Channels
     Channels() {}
 
     /// Well known channel names
-    static const char *adminChannel, *gpioChannel, *serialChannel;
+    static const char *adminChannel, *gpioChannel, *serialChannel, *mqttChannel;
 
     const meshtastic_ChannelSettings &getPrimary() { return getByIndex(getPrimaryIndex()).settings; }
 
@@ -139,4 +139,4 @@ class Channels
 };
 
 /// Singleton channel table
-extern Channels channels;
+extern Channels channels;