Trigger firmware update via MQTT (ESP Only). (#953)

If enabled in user_config.h this will trigger the OMG to update it's firmware from the link
provided in the MQTT message.

Example message:
{\"version\":\"test\",\"password\":\"OMGPASSWORD\",\"url\":\"https://github.com/1technophile/OpenMQTTGateway/releases/download/v0.9.6/esp32dev-ble-cont-firmware.bin\"}

If the firmware version in the message is not the same as the current OMG version the device will self update using the link in the url of the message.

docs/use/gateway.md

@@ -33,6 +33,26 @@ If you want the settings to be kept upon gateway restart, you can publish the co
 Auto discovery is enable by default on release binaries, on platformio (except for UNO). With Arduino IDE please read the [advanced configuration section](../upload/advanced-configuration#auto-discovery) of the documentation.
 :::
 
+# Firmware update from MQTT (ESP only)
+
+The gateway can be updated through an MQTT message by providing a JSON formatted message with a version number, OTA password (optional, see below), and URL to fetch the update from.  
+
+To enable this functionality, `MQTT_HTTPS_FW_UPDATE` will need to be defined or the line that defines in in user_config.h will need to be uncommented.
+
+::: tip
+If using an unsecure MQTT broker it is **highly recommended** to disable the password checking by setting the macro `MQTT_HTTPS_FW_UPDATE_USE_PASSWORD` to 0 (default is 1 (enabled)), otherwise a clear text password may be sent over the network.
+:::
+
+### Example firmware update message:
+```
+mosquitto_pub -t "home/<gateway_name>/commands/firmware_update" -m
+'{
+  "version": "test",
+  "password": "OTAPASSWORD",
+  "url": "https://github.com/1technophile/OpenMQTTGateway/releases/download/v0.9.6/esp32-m5stack-ble-firmware.bin"
+}'
+```
+
 # State LED usage
 
 The gateway can support up to 3 LED to display its operating state: