Add AgentsController, policy and feature flag

What does this MR do and why?

Adds routes, controller and basic view for Ai Agents feature. Adding models is part of a parallel work along this one #437299 (closed), so the index doesn't show anything as of now. Licensing check will also be added in a follow up.

MR acceptance checklist

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

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

Example below:

1. Enable the feature flag

   Feature.enable(:agent_registry)

2. Navigate to /-/ml/agents

Related to #437299 (closed)