Support Cluster Agent ID in Environment Create mutation

This MR is based on Support Cluster Agent ID in Environment Update ... (!121115 - merged)

What does this MR do and why?

Support Cluster Agent ID in Environment Create mutation

This commit allows users to set Cluster Agent ID to a newly created environment.

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

Mutation request examples

Set an cluster to an environment

GraphQL Mutation request

mutation {
  environmentCreate(input: {
    	projectPath: "root/pipeline-playground",
    	name: "test",
      clusterAgentId: "gid://gitlab/Clusters::Agent/2"
    }) {
    environment {
      name
      clusterAgent {
        name
      }
    }
    errors
  }
}

Response

{
  "data": {
    "environmentCreate": {
      "environment": {
        "name": "test",
        "clusterAgent": {
          "name": "prod"
        }
      },
      "errors": []
    }
  }
}

How to set up and validate locally

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

MR acceptance checklist

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