allow downloading datasets as JSON, minor refactor

[dinmukhamedm]

[!IMPORTANT] Add JSON dataset download feature and refactor download logic with a reusable DownloadButton component.

• New Feature:
  • Add GET endpoint in route.ts to download datasets as JSON, checking user authorization and dataset existence.
• Components:
  • Add DownloadButton component in download-button.tsx for reusable download functionality.
  • Replace existing download logic in evaluation.tsx with DownloadButton for CSV downloads.
  • Add DownloadButton to dataset.tsx for JSON downloads.
• Refactor:
  • Remove old CSV download logic from evaluation.tsx.

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