OpenAPI extensions for AEP-0203 - field behavior

[toumorokoshi]

          > It's unclear to me if we need an OpenAPI extension, but I'd guess we do?

We definitely need OpenAPI variants - I'll file an issue at least.

Originally posted by @toumorokoshi in https://github.com/aep-dev/aep.dev/issues/170#issuecomment-2067802797