Implement `@oneOf` input unions

[goto-bus-stop]

What will we need, beyond validation?

graphql/graphql-spec#825

[SimonSapin]

Based on a first skim of the proposed spec:

• A new built-in directive definition
• Some new schema validation rules
• A new rule for input coercion
  • In validation, for literal object values
  • In the execution engine, for variables (even though introspection, the only user of this engine for now, does not use @oneOf)
• A new schema-introspection field

[mmuth]

This would be a very great enhancement for the whole apollo stack, as currently it can't be used with rover and other federation tools. On the other hand, e.g. apollo client kotlin already supports it but requests oneOf related properties when performing introspection - which makes it incompatible with the current rover implementation.