Transition to new flat JSON format

[soininen]

This is a companion PR to spine-tools/Spine-Database-API#539. It contains changes needed in Toolbox to get things working with the new parameter value JSON format.

You need the WIP-data-transition branches from Spine-Database-API and spine-items to test and develop this.

Re #2506

Checklist before merging

• Documentation is up-to-date
• Release notes have been updated
• Unit tests have been added/updated accordingly
• Code has been formatted by black & isort
• Unit tests pass

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.80%. Comparing base (d28dd46) to head (2a5baa6).

Files with missing lines	Patch %	Lines
spinetoolbox/widgets/custom_qtableview.py	54.54%	5 Missing ⚠️
spinetoolbox/spine_db_manager.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3170      +/-   ##
==========================================
- Coverage   68.82%   68.80%   -0.02%     
==========================================
  Files         198      198              
  Lines       38303    38305       +2     
  Branches     4586     4586              
==========================================
- Hits        26361    26356       -5     
- Misses      10635    10641       +6     
- Partials     1307     1308       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.