feat!: improve deserialization and production logic

[JKRhb]

This PR reworks those parts of the library that are responsible for deserializing Thing Descriptions (creating ThingDescription class objects in the process) and for producing ExposedThing instances.

The validation of TDs is now “baked in” more directly into the deserialization process, making it possible to get rid of the JSON Schema based validation that was used before, making the instantiation of a ThingDescription object a bit more lightweight.

There are a few edge cases that aren't fully covered yet at the moment (especially when it comes to top-level forms and the permitted operations there), so these aspects will be re-revisited in follow-up PRs.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.12281% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 58.50%. Comparing base (39ccee8) to head (91669cb).

Files	Patch %	Lines
...b/src/core/definitions/extensions/json_parser.dart	98.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
- Coverage   66.04%   58.50%   -7.54%     
==========================================
  Files          78       77       -1     
  Lines        2453     2010     -443     
==========================================
- Hits         1620     1176     -444     
- Misses        833      834       +1     

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