Add Agent List to Cluster List View

What does this MR do?

We are in the process of building a new GitOps agent for GitLab. See &3329

As part of this process, we have a new design to display the list of agents on the cluster list page. See: #228845 (closed)

This MR will:

• Add agent list vues and apollo query the ee folder (This feature is only available for premium customers)
• Add helper methods to display the new design (The design only displays for premium customers, on a project page)
• Update haml files to use the new helper methods, vue components, and new design
• Adde new feature flag cluster_agent_list, so it can be optionally disabled
• Update associated tests and translations

Screenshots

Tabs Design

Core View, Group View, and Admin view

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Closes #228845 (closed)