Draft: Add support for GitLab slash command

What does this merge request do and why?

This MR adds support to call GitLab /explain command via GraphQL endpoint.

• A new model called slash_explain.
• Requires new version of code_explanation_v4.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

Relates to #153