Add union type redirect

[paulpdaniels]

Adds the ability to redirect a type union to to a different type. This allows you to reuse types between unions like:

sealed trait MyUnion
object MyUnion {
  @GQLValueType
  case class Redirect(value: UserProfile) extends MyUnion
  case class Error(msg: String) extends MyUnion
}

// Doesn't need to be part of the sealed trait anymore so can be reused for other types.
case class UserProfile(id: String, email: String)

producing a graphql union with the Redirect type removed and the UserProfile treated as part of the union instead.

type UserProfile {
  id: String!
  email: String!
}

type Error {
  msg: String!
}


union MyUnion = UserProfile | Error

[ghostdogpr]

Sorry, got a couple hectic days at work, will try to review tomorrow!

[ghostdogpr]

Looks good!

Can you add some docs (I don't mind if you do it in another PR so I can merge it when I release)?

Also, can you fix the example in the PR, it doesn't seem complete (no annotation and the type names don't match). Could be useful if someone stumble upon it.

[paulpdaniels]

@ghostdogpr done!