Capture and write final prompt to output BQ table

What does this merge request do and why?

This MR tries to capture the final prompt sent to the model. This crucial information for debugging and improvements.

• Modify the get_batch_completion interface to return both the response as well as the final prompt (json payload).
• Modify all the affected places where get_batch_completion interface is used.
• Write the final prompt to output BQ table.

Ref: #180 and https://gitlab.com/gitlab-org/modelops/ai-model-validation-and-research/ai-evaluation/prompt-library/-/work_items/190

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.