Show associated agent in the environment settings

What does this MR do and why?

As part of allowing to select an agent in the Environment page, this MR shows the associated agent in the environment settings page. The change is behind the environment_settings_to_graphql feature flag. I recently updated the page to get environment details from GraphQl but didn't realize we can't add a new field to the query in the same milestone as the backend part was merged. In this MR I put the old logic back and only do the new GraphQl logic when the feature flag is enabled.

Screenshots or screen recordings

How to set up and validate in review app

1. Please see the environment I've set up for this MR: https://gitlab-review-396407-all-25ngz8.gitlab-review.app/gitlab-qa-sandbox-group-2/cluster-agent/-/environments/1/edit

How to set up and validate locally

1. Enable the environment_settings_to_graphql feature flag

         Feature.enable(:environment_settings_to_graphql)

2. Visit the Environments page:

   • Project -> Operate -> Environments if the new navigation is enabled
   • Project -> Deployments -> Environments if the new navigation is not enabled

3. Create/Edit an environment using the UI

4. To create an agent that can appear in the agent selector:

   1. Enable kas_user_access feature flag

      Feature.enable(:kas_user_access)

   2. Enable expose_authorized_cluster_agents feature flag

      Feature.enable(:expose_authorized_cluster_agents)

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

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

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

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

   6. 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 (Optional) Deploy the GitLab Agent (agentk) with k3d section to create a local cluster and connect your agent with the cluster.

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

6. Return to the Edit environment page to verify the associated agent name is show on the agent selector.

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:

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

Related to #396407 (closed)