Expose cluster agent of the environment

This MR is on top of Add cluster_agent_id FK to environments (!120954 - merged)

What does this MR do and why?

This MR exposes registered cluster agent for the environment via GraphQL.

Users can't register a cluster agent yet. It'll be followed up after Environment Update GraphQL Mutation (!120561 - merged).

Related to Allow to Select Agent in Environment setting page (#396407 - closed)

Screenshots or screen recordings

Query

query {
  project(fullPath: "root/pipeline-playground") {
    environments {
      nodes {
        id
        name
        clusterAgent {
          name
        }
      }
    }
  }
}

Response

{
  "data": {
    "project": {
      "environments": {
        "nodes": [
          {
            "id": "gid://gitlab/Environment/17",
            "name": "production",
            "clusterAgent": {
              "name": "prod"
            }
          }
        ]
      }
    }
  }
}

How to set up and validate locally

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

MR acceptance checklist

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

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