Add PipelineMessageType to GraphQL

What does this MR do and why?

This is a follow-up that is part of this epic: &6859 and this issue: #346578 (closed)

#350276 (closed)

Screenshots or screen recordings

{
  project(fullPath: "root/downstream-pipeline-project") {
    pipeline(iid: "37") {
      warningMessages {
          content
          id
      }
    }
  }
}

{
  "data": {
    "project": {
      "pipeline": {
        "warningMessages": [
          {
            "content": "root `types` is deprecated in 9.0 and will be removed in 15.0 - read more: https://docs.gitlab.com/ee/ci/yaml/#deprecated-keywords",
            "id": "gid://gitlab/Ci::PipelineMessage/7"
          },
          {
            "content": "jobs:joba `type` is deprecated in 9.0 and will be removed in 15.0 - read more: https://docs.gitlab.com/ee/ci/yaml/#deprecated-keywords",
            "id": "gid://gitlab/Ci::PipelineMessage/8"
          }
        ]
      }
    }
  }
}

MR acceptance checklist

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