@requires on Union field

[rickbijkerk]

Issue Description

GIven a schema like the following i cant seem to figure out the correct syntax to add a requires on field thats a union

type Offer@key(fields: "uuid") {
  uid: ID!

  testField: String! @requires(fields: "condition { ... on SecondHandCondition { description } } }")
  testField: String! @requires(fields: "condition { description } }")
  # what is the correct syntax here?

  condition: OfferCondition! @external 
}

union OfferCondition = SecondHandCondition | NewCondition

type NewCondition{
  description: String
}

type SecondHandCondition{
  description: String!
}

Link to Reproduction

none

Reproduction Steps

If above example is not sufficient i can provide a reproduction, as of right now it feels like a lacking feature/documentation?