Add pipeline to create code explanation dataset

What does this merge request do and why?

This MR introduces a new pipeline to create code-explanation dataset using gitlab codebase.

• Read raw code files from https://console.cloud.google.com/bigquery?authuser=0&project=dev-ai-research-0e2f8974&ws=!1m5!1m4!4m3!1sdev-ai-research-0e2f8974!2scode_suggestion!3sinput_raw_v1
• Extract whole functions and classes from the raw code
• Store the code snippet in BQ as our code-explanation dataset.

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.

Closes #131 (closed)