Improved ability to register arbitrary MQTT message payload as a channel message parameter.
Previously when you set "Channel message composition" to "MQTT payload as parameter value", there were no options on how to parse it, only JSON syntax was used. Thus, to store string values it was required to enclose them into double quotes.
Now there is an option "MQTT payload data type for parameter" with 3 variants: "JSON value" (the default, denotes previous behavior), "Plaintext string" to store an utf8-valid string payload value, and "Binary/HEX data" to store arbitrary payload in HEX encoding.
