Refactoring: Introducting JsonSerializable

[forman]

Refactoring: Making most of the data classes JSON-serializable using JsonSerializable instead of the very naive ToDictMixin.

Checklist (strike out non-applicable):

• Changes documented in CHANGES.md
• Related issue exists and is referred to in the PR description and CHANGES.md
• Added docstrings and API docs for any new/modified user-facing classes and functions
• Changes/features documented in docs/*
• Unit-tests adapted/added for changes/features
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.26%. Comparing base (b1596ff) to head (1be1aec).
Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #17      +/-   ##
==========================================
+ Coverage   95.92%   96.26%   +0.34%     
==========================================
  Files          52       51       -1     
  Lines        2060     2061       +1     
  Branches      269      269              
==========================================
+ Hits         1976     1984       +8     
+ Misses         47       44       -3     
+ Partials       37       33       -4     

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