refactor: move API handlers folder structure

[jfcalvo]

Description

This PR include changes moving API handlers to follow this structure:

/api
  /handlers
    /v1
      records.py
      questions.py
      ...

The behavior has not been changed.

Refs #4868

Type of change

(Please delete options that are not relevant. Remember to title the PR according to the 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

(Please describe the tests that you ran to verify your changes. And ideally, reference tests)

• Test should be passing as before.

Checklist

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

[frascuchon]

The only comment here is to remove changes to the argilla project. By fixing it to server version ~=1.29.0, all this change won't affect it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.38%. Comparing base (bb1b41b) to head (a41aae5).

❗ Current head a41aae5 differs from pull request most recent head df7f0ce

Please upload reports for the commit df7f0ce to get more accurate results.

Additional details and impacted files

@@                              Coverage Diff                               @@
##           refactor/api-folder-structure-improvements    #4955      +/-   ##
==============================================================================
- Coverage                                       91.39%   91.38%   -0.01%     
==============================================================================
  Files                                             136      136              
  Lines                                            5831     5829       -2     
==============================================================================
- Hits                                             5329     5327       -2     
  Misses                                            502      502              

Flag	Coverage Δ
argilla-server	91.38% <100.00%> (-0.01%)	⬇️

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