feat: enable model selection for evals

What does this merge request do and why?

This MR enables model selection support for evals using CEF.

Closes gitlab-org/modelops/ai-model-validation-and-research/ai-evaluation/prompt-library#802

How to set up and validate locally

Please, find the instructions in https://gitlab.com/gitlab-org/modelops/applied-ml/code-suggestions/ai-assist/-/blob/07765c572f454d55a8dbd1b2558e326ce63d05b5/docs/model_selection.md#model-selection-for-evaluation-with-cef.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.
• If this change requires executor implementation: verified that issues/MRs exist for both Go executor and Node executor or confirmed that changes are backward-compatible and don't break existing executor functionality.