[SUBSCRIBER] Set up MQTT subscriber service

[ezehf2]

Create subscriber.py in the mqtt/ folder. The subscriber must connect to the HiveMQ broker using the settings in config.json — do not hardcode the broker address or port. Once connected, it should subscribe to all topics defined in config.json and print every received message to the console to confirm it is working. Test by running Dara's publisher at the same time and confirming messages are received. Commit with message [SUBSCRIBER] Add MQTT subscriber service and reference this issue. Do not start until config.json is in the repo.