Feat: Improve Placeholders task

[adrien-berchet]

Description

For some applications it's simpler to work with JSON than CSV files, so this PR adds JSON export.
It also changes the configuration format to make it easier to compute the placeholders for several regions / mtype couples.

This PR requires BlueBrain/NeuroM#1080

Checklist

This pull request is:

• A documentation / typographical error fix
  • Good to go, no issue or tests are needed
• A short code fix
  • Please include: Fixes: #<issue number> in the description if it solves an existing issue
    (which must include a complete example of the issue).
  • Please include tests that fail with the main branch and pass with the provided fix.
• A new feature implementation or update an existing feature
  • Please include: Fixes: #<issue number> in the description if it solves an existing issue
    (which must include a complete example of the feature).
  • Please include tests that cover every lines of the new/updated feature.
  • Please update the documentation to describe the new/updated feature.

[codecov]

Codecov Report

Merging #110 (b68181a) into main (8098f7f) will increase coverage by 0.01%.
The diff coverage is 86.20%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
+ Coverage   84.40%   84.42%   +0.01%     
==========================================
  Files          15       15              
  Lines        1796     1836      +40     
  Branches      267      280      +13     
==========================================
+ Hits         1516     1550      +34     
- Misses        184      186       +2     
- Partials       96      100       +4     

Flag	Coverage Δ
pytest	84.42% <86.20%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files Changed	Coverage Δ
src/morphology_workflows/placeholders.py	90.16% <81.81%> (-9.84%)	⬇️
src/morphology_workflows/tasks/placeholders.py	91.42% <90.90%> (-8.58%)	⬇️
src/morphology_workflows/utils.py	82.75% <92.30%> (+1.20%)	⬆️
src/morphology_workflows/tasks/cli.py	83.12% <100.00%> (+2.50%)	⬆️