fix: allow visible_labels=None & disallow fields/questions with the same name

[alvarobartt]

Description

This PR solves two minor fixes reported before the 1.9.0 release related to the validations to make sure that:

• visible_labels in LabelQuestion and/or MultiLabelQuestion can be set to None so that all the labels show in the Argilla UI
• The fields and questions of a FeedbackDataset cannot have the same name, otherwise, a ValueError should be raised

Closes #3034, #3123

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested

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

• Add unit tests for visible_labels=None to make sure it's kept and not replaced by default 20
• Add unit tests for FeedbackDataset init with duplicated field and question names

Checklist

• I have merged the original branch into my forked branch
• 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 have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[frascuchon]

Just a comment to show links properly in the CHANGELOG.md