Add support for @oneOf input objects

[steve-marmalade]

It looks like the community has landed on a solution for describing a "union of inputs" in a GraphQL schema: graphql/graphql-spec#825

I'd love to use this in my Ariadne server. Does that PR demonstrate enough momentum around the feature to consider implementing it?

[upcFrost]

Strawberry has already implemented it: https://github.com/strawberry-graphql/strawberry/pull/3429/files

[DamianCzajkowski]

Thank you for the idea! I'd be happy to implement this, and I'm adding it to my list of things to work on right now.

[upcFrost]

Can it be done with ariadne alone? From what i see it will require porting graphql/graphql-js#3513 to the python graphql-core lib (graphql-python/graphql-core#219), otherwise both the introspection and the validation will be broken