@oneOf directive support

[mengledowl]

The GraphQL working group recently upgraded the @oneOf directive RFC (graphql/graphql-spec#825) to RFC 2, which states:

GraphQL libraries should implement drafts to provide valuable feedback, though are encouraged not to enable the draft feature without explicit opt-in when possible.

This is one of the most anticipated (IMO) new additions to the spec, and the work has already been done for graphql-js (though unreleased as of yet).

I would like to propose that this feature be exposed in TypeGraphQL for those of us who want to have early access to it. I'm not sure what would be involved in that effort (if any), but wanted to get the conversation started.

[MichalLytek]

@Directive("@oneof")? 🤔