Add ai_services.enabled to enable AI Gateway and Duo Workflow Service

What does this merge request do and why?

This MR adds ai_services.enabled to enable both GitLab AI Gateway and Duo Workflow Service with a single setting. Before, each service had to be enabled separately. Individual services can still be overridden if only one is needed. To reflect this change, the setup_ai_gateway rake task is renamed to setup_ai_services since it now sets up both services.

Related to #3062

How to set up and validate locally

1. git checkout 3062-add-single-ai-gateway-duo-setting
2. Enable AI services:

   gdk config set ai_services.enabled true
   gdk reconfigure

3. Try overriding one service:

   gdk config set duo_workflow.enabled false
   gdk reconfigure

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This MR references an issue describing the change.
• This change is backward compatible. If not, include steps to communicate to users.
• Tests added for new functionality. If not, raise an issue to follow-up.
• Observability added/updated (logging, metrics, tracing).
• Documentation added/updated.
• Announcement added for notable changes.
• gdk doctor test added.