Forwarding directives to input types

[jakubknejzlik]

Currently it's possible to specify custom directives for fields. They are propagated to the final schema, but only for type fields. We need a way how to specify custom directives that need to be used for input fields (eg. for validation purposes).

Example:

// model.graphql
type User {
  email: String! @validation(email: true)
  age: Int @validation(min: 0) 
}

// gen/schema.graphql
input UserCreateInput {
  id: ID
  email: String! @validation(email: true)
  age: Int @validation(min: 0) 
}

input UserUpdateInput {
  email: String @validation(email: true)
  age: Int @validation(min: 0) 
}

The question is how to choose which directives propagate to type fields and which to create/update input fields (I'm not sure if it's ok to propagate them all).