Render related deployments from the Kustomization's inventory object

What does this MR do and why?

This MR adds a list of deployment resources related to the Kustomization. It only uses the Kustomization's inventory object, which provides the resource name, namespace, and kind. We don't show the status of the deployments in this MR.

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

How to set up and validate locally

Easily verify without setting up an agent

1. Visit the Environments page at Project -> Operate -> Environments page
2. Create an environment
3. Enable k8s_tree_view feature flag for this project:

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

4. Visit the Environment details page
5. Apply patch (copy it and pbpaste | git apply):

patch

diff --git a/app/assets/javascripts/environments/environment_details/components/kubernetes/kubernetes_overview.vue b/app/assets/javascripts/environments/environment_details/components/kubernetes/kubernetes_overview.vue
index 79469c931d1b..73ce7505f59b 100644
--- a/app/assets/javascripts/environments/environment_details/components/kubernetes/kubernetes_overview.vue
+++ b/app/assets/javascripts/environments/environment_details/components/kubernetes/kubernetes_overview.vue
@@ -118,6 +118,29 @@ export default {
     fluxResourceStatus() {
       return this.fluxKustomization?.conditions || this.fluxHelmReleaseStatus?.conditions;
     },
+    mockFluxKustomization() {
+      return {
+        kind: 'Kustomization',
+        metadata: {
+          name: 'flux-system',
+        },
+        conditions: [
+          {
+            message: 'Applied revision: main@sha1:f259306804c5b8417748b33c86fcd5a327ed1aba',
+            reason: 'ReconciliationSucceeded',
+            status: 'True',
+            type: 'Ready',
+          },
+        ],
+        inventory: [
+          { id: 'flux-system_failed-deployment_apps_Deployment' },
+          { id: 'flux-system_helm-controller_apps_Deployment' },
+          { id: 'flux-system_kustomize-controller_apps_Deployment' },
+          { id: 'flux-system_notification-controller_apps_Deployment' },
+          { id: 'flux-system_source-controller_apps_Deployment' },
+        ],
+      };
+    },
   },
   methods: {
     handleError(message) {
@@ -174,7 +197,7 @@ export default {
       v-model="activeTab"
       :configuration="k8sAccessConfiguration"
       :namespace="kubernetesNamespace"
-      :flux-kustomization="fluxKustomization"
+      :flux-kustomization="mockFluxKustomization"
       class="gl-mb-5"
       @cluster-error="handleError"
       @loading="podsLoading = $event"
diff --git a/app/assets/javascripts/environments/environment_details/index.vue b/app/assets/javascripts/environments/environment_details/index.vue
index f87723b33e9a..fa9a19621cef 100644
--- a/app/assets/javascripts/environments/environment_details/index.vue
+++ b/app/assets/javascripts/environments/environment_details/index.vue
@@ -46,7 +46,31 @@ export default {
         };
       },
       update(data) {
-        return data?.project?.environment;
+        const result = data?.project?.environment;
+        if (!result.clusterAgent) {
+          const clusterAgent = {
+            __typename: 'ClusterAgent',
+            id: 'gid://gitlab/Clusters::Agent/123',
+            name: 'my-agent',
+            webPath: '/gitlab-org/cluster-integration/-/cluster_agents/my-agent',
+            tokens: {
+              __typename: 'ClusterAgentTokenConnection',
+              nodes: [
+                {
+                  __typename: 'ClusterAgentToken',
+                  id: 'gid://gitlab/Clusters::AgentToken/123',
+                  lastUsedAt: '2023-05-30T11:47:56Z',
+                },
+              ],
+            },
+          };
+          return {
+            ...result,
+            clusterAgent,
+          };
+        }
+
+        return result;
       },
       result() {
         this.updateCurrentTab();

6. Verify the Tree view

Verify with real data

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 #463333 (closed)