Add rake task to set up AI gateway within GDK

What does this merge request do and why?

• This change adds a rake task to set up AI Gateway within GDK.
  • Prompts for Anthropic key
  • Sets up environment variables

Documentation will be added in a follow-up MR. Thought it'd be faster to review if rake task and documentation were in separate MRs.

Follow up MRs:

• !3810 (merged)
• gitlab!156970 (merged)

How to set up and validate locally

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

1. From gdk root directory, enter rake setup_ai_gateway
2. Go through prompts. For local validation, Anthropic API Key not required. Feel free to enter whatever key and check that it is saved in gitlab-ai-gateway/.env file
3. Check that localhost:5052/docs is up and running after script is complete.

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 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.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.