Add support for error headers in auth callout handlers

[istvan-andrasi]

Signed-off-by: Andrási István [email protected]
Resolves #7501

[neilalexander]

LGTM principally but would like @aricart to take a look too.

Also wondering if we should update the Auth Callouts ADR.

[ripienaar]

All significant content the server react on from the auth callout service must be signed and verifiable imo. If your service can't sign then thats something to log in the service logs. Conceptually I am not sure we can just start acting on unsigned/unverifiable information here

[istvan-andrasi]

Since it's just a small QoL improvement for the callout service developer, if it raises concerns on the server side, it's probably not worth it.

[ripienaar]

For background when we added the error feedback in the JWT my actual feature request was to have a header the server would just log - essentially exactly this.

But at the time we got the current behaviour instead for the reasons I mentioned :)