Include AI Gateway in GDK

Overview

Include AI Gateway in GDK, so that it's easier to run Duo Chat / CS locally for frontenders and non technical people.

See https://docs.gitlab.com/ee/development/ai_features/index.html#test-ai-features-with-ai-gateway-locally for the full details.

Impacted categories

The following categories relate to this issue:

• 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.