Add `manual_variables` field into PipelineType

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What does this MR do and why?

Issue: #323097

JiHu Issue: https://jihulab.com/gitlab-cn/gitlab/-/issues/3965

Add manual_variables fields to pipeline object in GraphQL.

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Before	After

How to set up and validate locally

1. Visit http://127.0.0.1:3000/-/graphql-explorer page
2. Execute the query statement (remember to replace the project id and pipeline iid)

{
  project(fullPath: "flightjs/Flight") {
    id
    pipeline(iid: 89) {
      manualVariables(first: 1) {
        nodes {
          key
          value
        }
      }
    }
  }
}