IOTData: update_thing_shadow() now properly handles complex nested deltas

[bblommers]

Fixes #8331

If there are other scenarios that I've missed, and the diff computation becomes even more complex, it might make sense to move away from jsondiff altogether, and add our own implementation. The whole point of jsondiff is to not have to deal with complexity like this. But for now, this will do.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.58%. Comparing base (60328d5) to head (4ea7561).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8342   +/-   ##
=======================================
  Coverage   94.58%   94.58%           
=======================================
  Files        1159     1159           
  Lines      101063   101086   +23     
=======================================
+ Hits        95587    95611   +24     
+ Misses       5476     5475    -1     

Flag	Coverage Δ
servertests	28.89% <10.71%> (-0.01%)	⬇️
unittests	94.55% <100.00%> (+<0.01%)	⬆️

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