Add support for setitem in inferencedata

[OriolAbril]

Description

Support idata[group] = dataset

Checklist

• Follows official PR format
• New features are properly documented (with an example if appropriate)?
• Includes new or updated tests to cover the new feature
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

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📚 Documentation preview 📚: https://arviz--2374.org.readthedocs.build/en/2374/

[codecov]

Codecov Report

Attention: Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 86.98%. Comparing base (5a928fb) to head (0c42cd1).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
arviz/data/inference_data.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2374      +/-   ##
==========================================
- Coverage   86.99%   86.98%   -0.01%     
==========================================
  Files         124      124              
  Lines       12862    12867       +5     
==========================================
+ Hits        11189    11193       +4     
- Misses       1673     1674       +1     

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