Add apache beam to registry and open source

This is the initial commit of the new billing system, rewritten as an Apache
Beam pipeline. This contains a basic end-to-end pipeline as proof of concept,
and boilerplate for GenerateInvoicesAction, which will eventually be our
automated invoice generation endpoint.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=174184171

java/google/registry/billing/GenerateInvoicesAction.java

@@ -0,0 +1,45 @@
+// Copyright 2017 The Nomulus Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package google.registry.billing;
+
+import static google.registry.request.Action.Method.POST;
+
+import google.registry.request.Action;
+import google.registry.request.auth.Auth;
+import javax.inject.Inject;
+
+/**
+ * Generates invoices for the month and stores them on GCS.
+ *
+ * <p>Currently this is just a stub runner that verifies we can deploy dataflow jobs from App
+ * Engine.
+ */
+@Action(
+    path = GenerateInvoicesAction.PATH,
+    method = POST,
+    auth = Auth.AUTH_INTERNAL_ONLY
+)
+public class GenerateInvoicesAction implements Runnable {
+
+  @Inject GenerateInvoicesAction() {}
+
+  static final String PATH = "/_dr/task/generateInvoices";
+
+  @Override
+  public void run() {
+    // MinWordCount minWordCount = new MinWordCount();
+    // minWordCount.run();
+  }
+}