Fix response and cleanup readme

[navarone-feekery]

Follow up from https://github.com/elastic/data-extraction-service/pull/12

• Restructure the response body to be more service-agnostic
• Cleanup readme and tests

Checklists

Pre-Review Checklist

• [x] this PR has a meaningful title
• [x] this PR links to all relevant github issues that it fixes or partially addresses
• [x] if there is no GH issue, please create it. Each PR should have a link to an issue
• [x] this PR has a thorough description
• [x] Covered the changes with automated tests
• [x] Tested the changes locally
• [x] Ran make e2e locally and all tests passed
• [x] Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)
• [x] Considered corresponding documentation changes
• [x] Contributed any configuration settings changes to the configuration reference