Modify fragment protocols to support deferred properties

[calvincestari]

This is the more general fragment work mentioned in the Generated models section of the RFC. This will probably have a dependency on #3139 being completed first.

Regardless of the fragment/field solution chosen all deferred fragment definitions in generated models __selections will get an additional property to indicate they are deferred. This helps to understand the models when reading them as well as being used by internal code.

// Updated Selection enum
public enum Selection {
  // other cases not shown
  case fragment(any Fragment.Type, deferred: Bool)
  case inlineFragment(any InlineFragment.Type, deferred: Bool)

  // other properties and types not shown
}

// Sample usage in a generated model
public class ExampleQuery: GraphQLQuery {
  // other properties and types not shown

  public struct Data: ExampleSchema.SelectionSet {
    public static var __selections: [ApolloAPI.Selection] { [
      .fragment(EntityFragment.self, deferred: true),
      .inlineFragment(AsEntity.self, deferred: true),
    ] }
  }
}