feat!: implement proper serialization logic

[JKRhb]

So far, there hasn't been a proper serialization implementation in place, which has been a major obstacle for adding ExposedThing functionality to the package. This PR changes that, making it possible to convert a Thing Description into a JSON (or CBOR) object.

The go-to approach for this kind of task in the Dart ecosystem is probably the json_serializable package. However, it seemed to me as if the annotation-based approach which is commonly used by that package (which also involves a generation step that I really dislike) is a bit too inflexible for the TD data model.

After the merge of this PR and maybe once the json_serializable has switched to using the new macros in the Dart language instead of their conventional builder-approach, we could consider replacing the (in some places somewhat inelegant implementation) with a nicer one that is close to the state of the art.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 99.63504% with 1 line in your changes missing coverage. Please review.

Project coverage is 63.38%. Comparing base (e1040ff) to head (7755c3e).

Files	Patch %	Lines
lib/src/binding_coap/coap_client.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #178      +/-   ##
==========================================
+ Coverage   58.50%   63.38%   +4.87%     
==========================================
  Files          77       78       +1     
  Lines        2010     2272     +262     
==========================================
+ Hits         1176     1440     +264     
+ Misses        834      832       -2     

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[JKRhb]

The implementation is not perfect yet, but I will already merge it to move with this topic and pave the way for more interesting features.

In any case, adding this implementation had the benefit of uncovering a few bugs, which I was able to fix – codecov really incentivized me to also go for an almost complete coverage of the changes :)