Draft: feat: add cli param to run dataset splits

What does this merge request do and why?

Resolves #22

TODO:

• Add class for only existing splits
• See what happens running multiple
• Change name to singular?

How to set up and validate locally

Existing splits are listed at the top of the code-suggestions-input-testcases-v1 dataset:

poetry run eli5 code-suggestions evaluate \
  --dataset="code-suggestions-input-testcases-v1" \
  --source=gitlab \
  --limit=10 \
  --offset=0 \
  --experiment-prefix=splits-test \
  --intent=completion \
  --rate-limit=29 \
  --splits=lang=php

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.