Add downstream_pipeline field to JobType

What does this MR do and why?

This is needed in order to unblock: !76853 (merged)

query:

{
  project(fullPath: "root/downstream-pipeline-project") {
    pipeline(iid: "13") {
      jobs {
        nodes {
          downstreamPipeline {
            id
            path
          }
        }
      }
    }
  }
}

response:

{
  "data": {
    "project": {
      "pipeline": {
        "jobs": {
          "nodes": [
            {
              "downstreamPipeline": {
                "id": "gid://gitlab/Ci::Pipeline/213",
                "path": "/root/downstream-pipeline-project/-/pipelines/213"
              }
            },
            {
              "downstreamPipeline": null <- this is a build
            }
          ]
        }
      }
    }
  }
}

How to set up and validate locally

In a project with a downstream pipeline, use the GraphQL query posted above. Most simple config to set this up (within the same project):

.gitlab-ci.yml

child-pipeline.yml

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.