Add support for @oneOf input objects

[DamianCzajkowski]

Discussed in #1193

^{Originally posted by steve-marmalade August 21, 2024}
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?