Expose work item iid for requirement type on GraphQL

What does this MR do and why?

Exposes work item iid on requirement type for GraphQL.

related to #382143 (closed)

How to set up and validate locally

1. Create a requirement on a project using the UI
2. Execute the following GraphQL query:

query { 
  project(fullPath: "PROJECT_PATH") {
    requirements {
      edges {
        node {
          workItemIid
        }
      }
    }
  }
}

MR acceptance checklist

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