Add mqtt_uns_structure configuration and uninstall functionality (#196)

- Prompt user for mqtt_uns_structure during installation and update src/config.py accordingly.
- Implement uninstall function in remote_install.sh to remove the rpi-mqtt-monitor directory, cron job, and systemd service.
- Update rpi-cpu2mqtt.py to utilize mqtt_uns_structure for state topics, ensuring proper MQTT topic structure.
- Modify config.py.example to include mqtt_uns_structure with a default empty value.

Co-authored-by: Sam Blackman <sam@samuelblackman.com>

src/config.py.example

@@ -10,6 +10,7 @@ mqtt_password = "password"
 mqtt_port = "1883"
 mqtt_discovery_prefix = "homeassistant"
 mqtt_topic_prefix = "rpi-MQTT-monitor"
+mqtt_uns_structure = ""
 
 # Retain flag for published topics
 retain = True