Added generate test file AI experiment

What does this MR do and why?

Created generate test file AI experiment. This adds a button to the diff file actions dropdown that will open a drawer and generate a test file for that diff file.

Screenshots or screen recordings

Screen_Recording_2023-04-21_at_15.33.11

How to set up and validate locally

• Enable openai_experimentation feature flag
• Enable the generate_test_file feature flag
• Have a project/instance at ultimate tier
• Setup OpenAI API key in GitLab settings
• Visit a merge request diff
• Click actions dropdown
• Click generate test file

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

#407778 (closed)