feat!: improve TD data model and serialization behavior

[JKRhb]

This PR reworks the serialization and general data models used for Thing Descriptions, and improves the code coverage. Due to the rework, a couple of bugs are fixed, especially regarding JSON Schema validation and URI variables. However, especially regarding the latter, some more work is still needed.

One important piece for simplifying the data models is introducing a new AugmentedForm class where most of the logic from interaction-affordance-specific forms is being moved while the data model classes are labeled as @immutable. This should make the library a bit more reliable and should also slightly improve performance due to the use of const constructors.

[codecov-commenter]

Codecov Report

Attention: 17 lines in your changes are missing coverage. Please review.

Comparison is base (61e4a32) 59.61% compared to head (273d57e) 64.10%.

Files	Patch %	Lines
lib/src/core/consumed_thing.dart	54.83%	14 Missing ⚠️
lib/src/binding_mqtt/mqtt_client.dart	0.00%	2 Missing ⚠️
lib/src/binding_http/http_client.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #89      +/-   ##
==========================================
+ Coverage   59.61%   64.10%   +4.49%     
==========================================
  Files          70       71       +1     
  Lines        2387     2385       -2     
==========================================
+ Hits         1423     1529     +106     
+ Misses        964      856     -108     

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