Rework clean up of ConsumedThing

[JKRhb]

This PR reworks the relationship between Servient, ConsumedThing, and the Subscriptions created when observing a property or subscribing to an event. It also removes the ConsumedThing cleanup code from the Servient class, which is not needed anymore now.

These changes decrease the coupling between the different parts of the codebase, making it overall more maintainable.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 5.55556% with 17 lines in your changes are missing coverage. Please review.

Project coverage is 66.74%. Comparing base (37fbb28) to head (aab9e9b).

Files	Patch %	Lines
lib/src/core/implementation/consumed_thing.dart	0.00%	12 Missing ⚠️
...ore/protocol_interfaces/protocol_subscription.dart	0.00%	3 Missing ⚠️
lib/src/binding_coap/coap_subscription.dart	0.00%	1 Missing ⚠️
lib/src/binding_mqtt/mqtt_subscription.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #162      +/-   ##
==========================================
+ Coverage   66.36%   66.74%   +0.38%     
==========================================
  Files          76       77       +1     
  Lines        2453     2436      -17     
==========================================
- Hits         1628     1626       -2     
+ Misses        825      810      -15     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.