files_external: Add OpenAPI spec

[provokateurin]

Summary

From #36666. Adds the necessary annotations and descriptions.

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)

[provokateurin]

@artonge @icewind1991 Can you help me with the CI failure? I don't see why my changes would cause any failure since they only add annotations which are completely ignored at runtime, thus no behavioral changes were made afaict

[artonge]

I think some of those tests are triggered only when some specific files are edited.
They might be failing for a long time 🙈 .
As your changes are mostly comments, I think we can merge without further concern.
If @icewind1991 agrees, feel free to merge :)

[provokateurin]

Yeah I already imagined something like that. There is no way the comments can break something

[icewind1991]

S3 external test fix is here: #39367