add createUser mutation

app/graphql/inputs/user_attributes_input.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+module Inputs
+  class UserAttributesInput < Types::BaseInputObject
+    graphql_name "UserAttributesInput"
+
+    argument :first_name, String, required: true
+    argument :last_name, String, required: true
+  end
+end