Add request field to Domain and Vulnerability endpoints

[JCantu248]

🗣 Description

Added the request field to the Domain and Vulnerability field, which contains a JSON list of their respective objects. Also updated the Pytest cases to ensure the endpoint still works as expected.

💭 Motivation and context

Change is required since the frontend expects the response in the form of a request: [] object.

🧪 Testing

Test cases were updated in response to these changes.

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced
  in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated
  to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.

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