[ENHANCEMENT] add custom field mode support

[burtenshaw]

Description

Closes #<issue_number>

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactor (change restructuring the codebase without changing functionality)
• Improvement (change adding some improvement to an existing functionality)
• Documentation update

How Has This Been Tested

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[frascuchon]

And maybe it makes sense to include here all required changes in the frontend part cc @damianpumar @leiyre

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.15%. Comparing base (7703b1d) to head (1350c4d).
Report is 1 commits behind head on feat/custom-field-support.

Additional details and impacted files

@@                      Coverage Diff                      @@
##           feat/custom-field-support    #5526      +/-   ##
=============================================================
- Coverage                      91.16%   91.15%   -0.02%     
=============================================================
  Files                            145      145              
  Lines                           5990     5993       +3     
=============================================================
+ Hits                            5461     5463       +2     
- Misses                           529      530       +1     

Flag	Coverage Δ
argilla-server	91.15% <100.00%> (-0.02%)	⬇️

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[burtenshaw]

And maybe it makes sense to include here all required changes in the frontend part cc @damianpumar @leiyre

Should we merge these SDK changes on to #5422, and then make the frontend changes there?