Add a Tree item component for Kubernetes tree view

What does this MR do and why?

This MR adds a TreeItem component for the Kubernetes tree view. Currently, it's only used for showing a Kustomization resource, but it can be used for every next resource to keep the stylings clean.

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

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Before	After

How to set up and validate locally

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

Prerequisites:

• NGINX, loopback interface, and HTTPS set up on your GDK
• Docker instance
• KAS enabled on GDK (steps 1-2 from the guide).

1. Follow this guide to start with the Flux.

2. Visit the Environments page at Project -> Operate -> Environments page

3. Create/Edit an environment using the UI

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

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

   • Select the "Connect a cluster" button
   • The modal should pop up
   • In the modal select "Select an agent or enter a name to create new"
   • You probably won't have any configured agents to show up in the list, create a new one by typing the name of your choice
   • The button should appear at the bottom of the list saying "Create agent: <your-agent-name>"
   • Select the button and click "Register" in the next view.
   • Save the token to use it in the next point.

6. Add the following configuration inside your project in .gitlab/agents/<your-agent-name>/config.yaml for the user_access agent:

   user_access:
     access_as: 
       agent: {}
     projects:
     - id: <your-group>/<your-project-to-share-agent-with>

7. Note that the shared agents should be connected to the cluster in order to appear in the list. Please follow points 3-8 from the guide and then the Deploy the GitLab Agent (agentk) with k3d section to create a local cluster and connect your agent with the cluster.

8. Select an agent from the dropdown in the Environments settings page and save the change.

9. The Flux resource selector will appear. Select Kustomization resource and save.

10. Visit the Environments page and select the environment with an associated agent and Flux resource.

11. Enable k8s_tree_view feature flag for this project:

Feature.enable(:k8s_tree_view, Project.find(<project id>))

12. Verify the Summary tab in the Kubernetes overview section.

Related to #463331 (closed)