Is your refactoring request related to a problem? Please describe.

Refactor the mqtt_service feature with the Cumulocity MQTT Service which should support a single MQTT connection for both SmartREST messages and free-form topics.

Describe the solution you'd like

Below describes the 4 items to refactor the current implementation of the mqtt_service integration in thin-edge.io. The key to the changes is to more think of the mqtt-service as an extension to the existing bridge, rather than a new type of bridge.

### Item 1: Remove username handling

The MQTT Service no longer requires setting the tenant id in the username. As the new MQTT Service now supports connecting to it exactly the same as the Core MQTT (except for the fact that it uses port 9883 instead of 8883).

Item 2: Remove the c8y.mqtt_service.url and c8y.mqtt_service.topic_prefix settings

Since the Cumulocity MQTT service will be supporting the SmartREST proxy, there is no longer a need for a dedicated url just for the Cumulocity MQTT service, as the native c8y.url or (c8y.http and c8y.mqtt) values can be set.

The c8y.mqtt_service.topic_prefix setting is also no longer needed as both the SmartREST topics and free-form topics will be under the mapper's prefix, e.g. c8y/.

Item 3: Remove default values for c8y.mqtt_service.topics

Remove the default values in the c8y.mqtt_service.topics settings so it does not create unnecessary subscriptions for the users.

Item 4: Refactor behaviour of the c8y.mqtt_service.enabled setting

When c8y.mqtt_service.enabled is set to true the following should happen:

• the default mqtt port should change from 8883 to 9883

• the topics defined in c8y.mqtt_service.topics should be added to the bridge's subscribed topics (to the cloud)

  remote: foo/bar
  local:  c8y/mqtt/in/foo/bar

• the bridge should create an outbound rule to map any topic under a given root topic to the mqtt service

  c8y/mqtt/out/#

  Where a message published

  local:  c8y/mqtt/out/foo/bar
  remote: foo/bar

Describe alternatives you've considered

Additional context