refactor: add DatasetPublishValidator class

[jfcalvo]

Description

A small refactor moving validation logic for publish_dataset context function to DatasetPublishValidator class. Once we merge this PR then I can do changes to the validations so we can increase flexibility to import datasets from the Hub.

I was bored of creating "helper functions" for counting queries for database so I have added count_by function into our database models mixin class. I hope it can be useful.

Type of change

• Refactor (change restructuring the codebase without changing functionality)

How Has This Been Tested

• Running test suite.

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/)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.28%. Comparing base (9677435) to head (4028784).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5568   +/-   ##
========================================
  Coverage    91.28%   91.28%           
========================================
  Files          145      145           
  Lines         6025     6037   +12     
========================================
+ Hits          5500     5511   +11     
- Misses         525      526    +1     

Flag	Coverage Δ
argilla-server	91.28% <100.00%> (+<0.01%)	⬆️

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