feat: add support to `-1` no label values for `ClassLabel` dataset features

[jfcalvo]

Description

This PR adds the following changes:

• Add support to ClassLabel features using -1 (no label) values and casting these values to be None.
• Now when importing fields, metadata, or suggestions and the value is None they will be ignored.

Refs https://github.com/argilla-io/roadmap/issues/21

Type of change

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

How Has This Been Tested

• [x] Adding new tests and modifying existing ones.

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## feat/add-hub-dataset-import #5607 +/- ## ============================================================== Coverage ? 91.18% ============================================================== Files ? 150 Lines ? 6253 Branches ? 0 ============================================================== Hits ? 5702 Misses ? 551 Partials ? 0 ``` | [Flag](https://app.codecov.io/gh/argilla-io/argilla/pull/5607/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io) | Coverage Δ | | |---|---|---| | [argilla-server](https://app.codecov.io/gh/argilla-io/argilla/pull/5607/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io) | `91.18% <100.00%> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io#carryforward-flags-in-the-pull-request-comment) to find out more.

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