build: Add OpenAPI fuzzer script

[provokateurin]

• Resolves: Use OpenAPI fuzzing to find mistakes in specifications and general errors in the code #47825

Summary

A script I used to fuzz Nextcloud APIs and find errors both in the backend and the specifications. It's not perfect, but definitely finds some real issues that can be fixed.

TODO

• Allow enabling extra apps, so they can be fuzzed too.
• Wait for [BUG] API accepts requests without authentication for endpoint with optional authentication schemathesis/schemathesis#3052 to be resolved and enable ignored_auth check again.
• Check if missing_required_header and unsupported_method can be enabled (and get handled properly in the backend)
• Maybe a better way to install schemathesis?

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)