Add new llm logger for debugging AI features

What does this MR do and why?

Adds a new logger and debugging statements for AI features.

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/409977

Screenshots or screen recordings

logging

How to set up and validate locally

1. tail -f log/llm.log
2. Set up your local instance for AI features
3. Test an AI feature like summarize comments
   1. Active it by enabling the feature flag: Feature.enable(:summarize_comments)
   2. Go to an issue with comments and press the "Summarize Comments" button (see video)

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.