Allow specifying channel index in JSON field "channel" for downlink

Still requires JSON message to be published to channel named "mqtt"

src/mqtt/MQTT.cpp

@@ -67,7 +67,9 @@ void MQTT::onReceive(char *topic, byte *payload, size_t length)
                         // construct protobuf data packet using TEXT_MESSAGE, send it to the mesh
                         meshtastic_MeshPacket *p = router->allocForSending();
                         p->decoded.portnum = meshtastic_PortNum_TEXT_MESSAGE_APP;
-                        p->channel = sendChannel.index;
+                        if (json.find("channel") != json.end() && json["channel"]->IsNumber() &&
+                            (json["channel"]->AsNumber() < channels.getNumChannels()))
+                            p->channel = json["channel"]->AsNumber();
                         if (json.find("to") != json.end() && json["to"]->IsNumber())
                             p->to = json["to"]->AsNumber();
                         if (jsonPayloadStr.length() <= sizeof(p->decoded.payload.bytes)) {
@@ -95,7 +97,9 @@ void MQTT::onReceive(char *topic, byte *payload, size_t length)
                         // construct protobuf data packet using POSITION, send it to the mesh
                         meshtastic_MeshPacket *p = router->allocForSending();
                         p->decoded.portnum = meshtastic_PortNum_POSITION_APP;
-                        p->channel = sendChannel.index;
+                        if (json.find("channel") != json.end() && json["channel"]->IsNumber() &&
+                            (json["channel"]->AsNumber() < channels.getNumChannels()))
+                            p->channel = json["channel"]->AsNumber();
                         if (json.find("to") != json.end() && json["to"]->IsNumber())
                             p->to = json["to"]->AsNumber();
                         p->decoded.payload.size =