Allow custom groups without warnings

[OriolAbril]

Description

Only emit a warning for custom groups when explicitly requested.

As InferenceData gets more uses, data related to the model but not specifically defined
in the schema is also stored as independent groups, which is perfectly fine and doesn't break
anything but it is annoying to get all the warnings or having to silence them constantly.
Moreover, this is set to not happen in arviz-base so it will also align behaviour.

Checklist

• Follows official PR format
• Includes a sample plot to visually illustrate the changes (only for plot-related functions)
• 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--2401.org.readthedocs.build/en/2401/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.81%. Comparing base (529d795) to head (a810882).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2401   +/-   ##
=======================================
  Coverage   86.81%   86.81%           
=======================================
  Files         124      124           
  Lines       12883    12884    +1     
=======================================
+ Hits        11184    11185    +1     
  Misses       1699     1699           

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