Add rich conditions support to MQTT monitor similar to DNS monitor,
allowing users to define flexible conditions on:
- topic: The MQTT topic that received the message
- message: The raw message content
- json_value: JSONata-extracted value from JSON payloads
This provides a more intuitive and powerful way to validate MQTT
messages compared to the basic keyword/json-query checks.
Maintains backward compatibility with existing keyword and json-query
check types.
Closes#5992
cf1391db6b