Show agent's information on Environment index page

What does this MR do and why?

This MR starts introducing the Environments page Kubernetes integration feature. It is the very first step that creates a new section, prepares to receive the agent data, and shows the agent details according to the design. The section should be shown only when the agent data was returned from the backend.

This MR provides the feature overview.

Screenshots or screen recordings

No agent data	Agent data available

How to set up and validate locally

1. Enable KAS on your GDK (steps 1-2 from the guide).

2. Visit the Project -> Infrastructure -> Kubernetes clusters page and create an agent following the instruction from the modal.

3. Create an environment.

4. The feature is currently behind a feature flag. Enable it by running:

   Feature.enable(:kas_user_access)

   and

   Feature.enable(:kas_user_access_project, Project.find(<your-project-id>))

5. Note that the backend isn't ready yet, in order to see the changes, please use the mock agent data in the agent computed of new_environment_item.vue and change it to the relevant data for your created agent:

   return {
      project: '<your-group>/<your-project>',
      id: '<agent-id>',
      name: '<agent-name>',
    };
   

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Review progress:

• UX review
• backend maintainer review
• frontend developer review
• frontend maintainer review

Related to #390769 (closed)