feat: Add metadata support for datasets

[jfcalvo]

Description

This PR add a new metadata column to datasets table and the following changes:

• When creating a dataset now it is possible to specify metadata values.
• When updating a dataset now it is possible to specify metadata values.
• Now metadata attribute is included on Dataset schema so the metadata of a dataset is exposed in our API.

Refs argilla-io/roadmap#21

Type of change

• New feature (non-breaking change which adds functionality)

How Has This Been Tested

• Adding new tests.

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 91.22%. Comparing base (6240d4f) to head (d9a65a5).
Report is 1 commits behind head on feat/argilla-direct-feature-branch.

Additional details and impacted files

@@                          Coverage Diff                           @@
##           feat/argilla-direct-feature-branch    #5586      +/-   ##
======================================================================
+ Coverage                               91.20%   91.22%   +0.01%     
======================================================================
  Files                                     145      145              
  Lines                                    6026     6037      +11     
======================================================================
+ Hits                                     5496     5507      +11     
  Misses                                    530      530              

Flag	Coverage Δ
argilla-server	91.22% <100.00%> (+0.01%)	⬆️

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