Support for the @oneOf directive

[tjarvstrand]

The RFC for the new @oneOf directive is now basically just a formality away from being accepted into the GraphQL spec.

It would be great to see type level support for it in graphql_codegen.

BTW, thanks for an awesome library!

[github-actions]

👋 @tjarvstrand
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[budde377]

@oneOf should now generate different factories for each field. Please check it out and let me know what you think.