GraphQL: Add upstream/downstream pipelines and source job to Pipelines

What does this MR do?

Adds the downstream and upstream pipeline information for a pipeline, plus project information.

Example query:

{
  project(fullPath: "root/test-b"){
    pipeline(iid: "13"){
     downstream{
        path
        iid
        project{
          id
          name
          fullPath
        }
        detailedStatus{
          group
          label
          icon
        }
        sourceJob{
          name
        }
      }
      upstream {
        iid
        path
        project{
          id
          name
          fullPath
        }
        detailedStatus {
          group
          label
          icon
        }
        sourceJob{
          name
        }
      }
  	}
  }
}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides

Relates to: #254236 (closed) / #268117 (closed)