Add support for GitLab Slash command

What does this merge request do and why?

This is MR is a replacement of !363 (closed) This MR adds support to call GitLab /explain, /tests and /refactor command via GraphQL endpoint.

• Add a new set of Slash command models.
• Require a new code explanation dataset that provides code, file_path, code_after and code_before.

How to set up and validate locally

1. Ensure GCP environment variables are setup.

2. Check out to this merge request's branch.

3. Copy the example config.

   cp data/config/duochat_eval_slash_code_explain_config.json{.example,}

4. Run the follow command to kick off the pipeline with a small sample.

   poetry run promptlib duo-chat eval --config-file=data/config/duochat_eval_slash_code_explain_config.json --test-run --sample-size 2

5. Verify the output CSVs containing good quality responses.

Merge request checklist

• I've ran the affected pipeline(s) to validate that nothing is broken.
• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.