Allow union match with base interface

[robbss]

Is your feature request related to a problem? Please describe.

When selecting a union type, it's not possible to match via a base interface, this prevents us from matching on Error to determine if any error happened.

Describe the solution you'd like
Given the schema:

interface Error {
  message: String!
}

type NotFoundError implements Error {
  message: String!
  code: String!
}

union UpdateTripError = NotFoundError

type UpdateTripPayload {
  trip: Trip
  errors: [UpdateTripError!]
}

I would like to be able to generate a query equivalent to:

updateTrip($input) {
  trip {
    comment
  }
  errors {
    ... on Error {
      message
    }
  }
}

graphClient.Mutation(q => q.UpdateTrip(..., o => new { Error = o.Errors(oo => oo.On<Error>().Select(ooo => ooo.Message)) }));

This is currently not possible, due to Error not being part of the union type UpdateTripError.

The query does work when queried directly against the GraphQL backend.