Expose humanTimeEstimate and humanTotalTimeSpent via graphql

What does this MR do?

Exposes humanTimeEstimate and humanTotalTimeSpent (for Issue type) in GraphQL.

• the fields are required by the frontend as part of #37102 (closed) (Sidebar: Time tracking should be a Vue app) and already exposed via our REST API.

Sample query

{
  project(fullPath: "gitlab-org/gitlab-test") {
    issues {
      nodes {
        humanTimeEstimate
        humanTotalTimeSpent
      }
    }
  }
}

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #37102 (closed)