Add DomainResource class to wrap MX lookup/normalize (#32864)

spec/lib/domain_resource_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe DomainResource do
+  describe '#mx' do
+    subject { described_class.new(domain) }
+
+    let(:domain) { 'example.host' }
+    let(:exchange) { 'mx.host' }
+
+    before { configure_mx(domain: domain, exchange: exchange) }
+
+    it 'returns array of hostnames' do
+      expect(subject.mx)
+        .to eq([exchange])
+    end
+  end
+end