Allow users to configure AI Gateway endpoint in the UI

Problem to solve

To configure an self-manage instance of GitLab to connect to a self-hosted model, an admin must setup a self-hosted AI Gateway, and point the GitLab instance to the created AIGW instance. This is currently done by setting an enviroment variable on GitLab. This is not ideal since it requires the GitLab instance to be restarted for the environment variable to picked up, and the configuration itself is opaque

Current proposal

Allow the AIGW endpoint to be set in the GitLab UI via the GitLab Duo features settings page.