[stable31] Add OpenAPI support

[backportbot]

Backport of #1018

Warning, This backport's changes differ from the original and might be incomplete ⚠️

Todo

• Review and resolve any conflicts
• Amend HEAD commit to remove the line stating to skip CI

---

Learn more about backports at https://docs.nextcloud.com/server/stable/go.php?to=developer-backports.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 38.28%. Comparing base (b267636) to head (81a6b07).
Report is 11 commits behind head on stable31.

Additional details and impacted files

@@            Coverage Diff            @@
##           stable31    #1028   +/-   ##
=========================================
  Coverage     38.28%   38.28%           
=========================================
  Files            33       33           
  Lines          1468     1468           
  Branches         79       79           
=========================================
  Hits            562      562           
  Misses          904      904           
  Partials          2        2           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.