Improve handling of defaults and serialization/deserialization of Thing Descriptions

[JKRhb]

This PR fixes a couple of issues I noticed when it comes to the (de)serialization behavior of Thing Descriptions. It also exposes an unnamed constructor for the ThingDescription class, which can have advantages in certain scenarios in comparison to the deserialization approach (from a Map<String, dynamic>).

As this PR contains some breaking changes, it will trigger the release of version 0.35.0.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.64%. Comparing base (3765571) to head (e698015).
Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #189      +/-   ##
==========================================
+ Coverage   63.40%   63.64%   +0.23%     
==========================================
  Files          78       78              
  Lines        2279     2294      +15     
==========================================
+ Hits         1445     1460      +15     
  Misses        834      834              

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