support @defer directive. (#255)

validator/prelude.graphql

@@ -27,6 +27,9 @@ directive @deprecated(reason: String = "No longer supported") on FIELD_DEFINITIO
 "The @specifiedBy built-in directive is used within the type system definition language to provide a scalar specification URL for specifying the behavior of custom scalar types."
 directive @specifiedBy(url: String!) on SCALAR
 
+"The @defer directive may be specified on a fragment spread to imply de-prioritization, that causes the fragment to be omitted in the initial response, and delivered as a subsequent response afterward. A query with @defer directive will cause the request to potentially return multiple responses, where non-deferred data is delivered in the initial response and data deferred delivered in a subsequent response. @include and @skip take precedence over @defer."
+directive @defer(if: Boolean = true, label: String) on FRAGMENT_SPREAD | INLINE_FRAGMENT
+
 type __Schema {
   description: String
   types: [__Type!]!

validator/schema.go

@@ -86,7 +86,7 @@ func ValidateSchemaDocument(ast *SchemaDocument) (*Schema, error) {
 			// scalars, it may (§3.13) define builtin directives. Here we check for
 			// that, and reject doubly-defined directives otherwise.
 			switch dir.Name {
-			case "include", "skip", "deprecated", "specifiedBy": // the builtins
+			case "include", "skip", "deprecated", "specifiedBy", "defer": // the builtins
 				// In principle here we might want to validate that the
 				// directives are the same. But they might not be, if the
 				// server has an older spec than we do. (Plus, validating this

validator/schema_test.go

@@ -20,6 +20,11 @@ func TestLoadSchema(t *testing.T) {
 		require.Equal(t, "Boolean", boolDef.Name)
 		require.Equal(t, ast.Scalar, boolDef.Kind)
 		require.Equal(t, "The `Boolean` scalar type represents `true` or `false`.", boolDef.Description)
+
+		deferDef := s.Directives["defer"]
+		require.Equal(t, "defer", deferDef.Name, "@defer exists.")
+		require.Equal(t, "if", deferDef.Arguments[0].Name, "@defer has \"if\" argument.")
+		require.Equal(t, "label", deferDef.Arguments[1].Name, "@defer has \"label\" argument.")
 	})
 	t.Run("swapi", func(t *testing.T) {
 		file, err := os.ReadFile("testdata/swapi.graphql")