Support for unsubscribing from topics

[el0y79]

Describe the feature

It seems atm the mqtt bridge does not support unsubscribing from topics once subscribed. I have to admit I am no expert in rust but looking at the code it looks to me like the unsubscribe has not been implemented yet. It only does some logging if Im not mistaken. This is in line with the behavior I have seen. When subscribing to a mqtt topic and later unsubscribing from that topic, messages are still being received for that topic.

[andrew-otiv]

I wrote a PR for this: #411