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.cpp

@@ -15,6 +15,7 @@ Channels channels;
 const char *Channels::adminChannel = "admin";
 const char *Channels::gpioChannel = "gpio";
 const char *Channels::serialChannel = "serial";
+const char *Channels::mqttChannel = "mqtt";
 
 uint8_t xorHash(const uint8_t *p, size_t len)
 {
@@ -313,4 +314,4 @@ bool Channels::decryptForHash(ChannelIndex chIndex, ChannelHash channelHash)
 int16_t Channels::setActiveByIndex(ChannelIndex channelIndex)
 {
     return setCrypto(channelIndex);
-}
+}