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/env/common/backend/WEB-INF/web.xml

@@ -57,6 +57,19 @@
     <url-pattern>/_dr/task/brdaCopy</url-pattern>
   </servlet-mapping>
 
+  <!-- Billing -->
+
+  <!--
+    Generates the invoice CSV for the month, which we send to billing to charge
+    registrars for their registrations.
+  -->
+  <servlet-mapping>
+    <servlet-name>backend-servlet</servlet-name>
+    <url-pattern>/_dr/task/generateInvoices</url-pattern>
+  </servlet-mapping>
+
+  <!-- ICANN Monthly Reporting -->
+
   <!--
     Monthly ICANN transaction and activity reports. This task generates report
     files (in CSV format) and stores them in GCS under