feat: add dataset split to be used along with row idx when `external_id` is not provided on mapping

[jfcalvo]

Description

This PR add the dataset imported split to be used as external_id when there is no value for external_id specified on the import mapping.

If importing the split train for a dataset and no external_id is provided the external_id will be calculated like the following:

• train_0: first row of train split.
• train_1: second row of train split.
• ...

With this we are avoiding row duplications when another split is imported to the same dataset. So if later we import the test split for the same dataset we will have for external_id:

• train_0: first row of train split.
• train_1: second row of train split.
• ...
• test_0: first row of test split.
• test_1: second row of test split.
• ...

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 additional 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[bot]]

Codecov Report

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

Project coverage is 91.19%. Comparing base (098d36b) to head (249e5d8). Report is 1 commits behind head on feat/argilla-direct-feature-branch.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## feat/argilla-direct-feature-branch #5616 +/- ## ====================================================================== + Coverage 91.18% 91.19% +0.01% ====================================================================== Files 150 150 Lines 6260 6261 +1 ====================================================================== + Hits 5708 5710 +2 + Misses 552 551 -1 ``` | [Flag](https://app.codecov.io/gh/argilla-io/argilla/pull/5616/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/5616/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io) | `91.19% <100.00%> (+0.01%)` | :arrow_up: | 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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