Add GraphQL type for agent metadata

What does this MR do and why?

Replaces a broken JSON field with a dedicated type for ConnectedAgent.metadata. Previously the metadata was not serialized properly, which always resulted in an empty value.

Screenshots or screen recordings

GraphQL request:

{
  project(fullPath: "path/to/agent/project") {    
    clusterAgents {
      edges {
        node {
          name
          connections {
            edges {
              node {
                connectedAt
                connectionId
                metadata {
                  version
                  commit
                  podName
                  podNamespace
                }
              }
            }
          }
        }
      }
    }
  }
}

Response:

{
  "data": {
    "project": {
      "clusterAgents": {
        "edges": [
          {
            "node": {
              "name": "my-agent",
              "connections": {
                "edges": [
                  {
                    "node": {
                      "connectedAt": "2021-09-15T09:40:29+10:00",
                      "connectionId": "8100640246091127",
                      "metadata": {
                        "version": "v14.2.2",
                        "commit": "2b22fa6",
                        "podName": "gitlab-agent-698f74d7c8-tnfp4",
                        "podNamespace": "gitlab-kubernetes-agent"
                      }
                    }
                  }
                ]
              }
            }
          }
        ]
      }
    }
  }
}

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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• I have evaluated the MR acceptance checklist for this MR.

Related to #322605 (closed)