Dangerbot for AI Gateway

What does this merge request do and why?

This MR adds Danger bot to the AI Gateway project, which is commonly used in GitLab projects for a various maintenance tasks, such as Reviewer roulette.

This tool mitigates the issues like https://gitlab.slack.com/archives/C051K31F30R/p1701803273449169 that the developers have no idea whom to ask for review in AI Gateway project.

See https://docs.gitlab.com/ee/development/dangerbot.html#enable-danger-on-a-project for more information.

Related to Document the process to be a maintainer / maint... (#356 - closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.