Add boilerplate code for "Group > Automate > Agents"

What does this MR do and why?

Add route, controller, menu item, and frontend code for Group > Automate > Agents. Behind the new feature flag ai_catalog_agents.

Similar to changes for flows in !209833 (merged).

🚧 🚧 🚧 NOTE: These issues are known and will be fixed in later MRs: Agents list not showing on group page, Empty state mentions "project", Toast message not showing group name, Group permissions for adminAiCatalogItemConsumer 🚧 🚧 🚧

References

Related to #580307, #580258 (closed), and #580944 (closed).

Screenshots or screen recordings

Screenshot	Screen recording
	[See NOTE above] Group agents recording.mov

How to set up and validate locally

1. Prerequisite: You have a Duo workflow-enabled project.
2. Enable the feature flags global_ai_catalog, and ai_catalog_agents.
3. Go to Explore > AI Catalog > Agents. For example, https://gdk.test:3443/explore/ai-catalog/agents.
   1. Create an agent. You should be redirected to an agent page.
   2. Select Enable in group. Select a Duo-enabled group.
4. Go to Group > Automate > Agents. For example, https://gdk.test:3443/groups/gitlab-duo/-/automate/agents.
   1. Verify that the agent is enabled for the group.

MR acceptance checklist

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