Add Group Level Cluster Agent View

What does this MR do and why?

Currently, we only show the Kubernetes cluster agents on the project level. This MR adds the existing view for the group level. While it uses the same components, it has some differences from the project view:

• no create agent flow, as the agent can only be created within a project;
• no shared agents, as they are currently unavailable on the group level;
• no agent configurations, as they are tight to the project.

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• #425969 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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

Before	After
certificate_based_clusters FF is disabled
404
certificate_based_clusters FF is enabled
n/a

How to set up and validate locally

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

Prerequisites:

• KAS enabled on GDK (steps 1-2 from the guide).

1. Visit Group -> Operate -> Kubernetes clusters page
2. Verify the empty state of the agents list
3. To add an agent, visit Project -> Operate -> Kubernetes clusters page
4. Create an agent following the instructions from the modal.
   • Select the "Connect a cluster" button
   • The modal should pop up
   • In the modal fill in the agent name with any unique name
   • Select "Create and register"
5. The agent will appear in the list
6. Visit Group -> Operate -> Kubernetes clusters page again
7. Verify the new agent is showing up in the list
8. Select the actions menu (3 dots in the table) and verify agent deletion functionality
9. Toggle the certificate_based_clusters feature flag to verify the changes work both with the feature flag enabled and disabled.

Related to #425969 (closed)