Improve AIGW setup

What does this merge request do and why?

• Moves away from env.runit, which is deprecated: #2133
• Uses a different ENV var for configured AIGW, see conversation in gitlab!204287 (closed)
• Sets up Duo Workflow Service alongside AI Gateway in setup script

Closes #3062 (closed)

How to set up and validate locally

Follow the instructions in the docs of this MR to setup ai services.

Notes:

• Ensure you do not have gitlab_ai_gateway.enabled/duo_workflow.enabled in your gdk.yml. If they are set to false, they will override ai_services.enabled and neither service will start.
• Use GDK_SELF_UPDATE=0 gdk rake setup_ai_services to setup ai services, this will prevent GDK from checking out the main branch.
• To use duo web chat, you must access gdk through the nginx proxy - default port 8080 for http.

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, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.