Adds script to debug self-hosted code suggestions setup

What does this MR do and why?

Adds a troubleshooting rake test to help administrators debug potential problems with their self-hosted installation

How to set up and validate locally

1. Make sure you have a valid ultimate license, and AI Gateway installed end running.
2. In the gitlab repo, the following rake task should succeed:

   export AI_GATEWAY_URL="http://gdk.test:5052"
   export CLOUD_CONNECTOR_SELF_SIGN_TOKENS=1
   
   bundle exec rake gitlab:duo:verify_self_hosted_setup

Related to #479128 (closed)