Support `@oneOf` directive

There is a new directive being introduced called `@oneOf`. the RFC is accepted and will soon be merged.

Would be great to have this supported in the plugin too.

See https://github.com/graphql/graphql-spec/pull/825/

<img width="1030" height="440" alt="Image" src="https://github.com/user-attachments/assets/86132dc5-fac0-4635-b73c-2dc9a6e43fd5" />