Add a "shell" pseudo-command to nomulus tool

Add the "shell" command which lets you run multiple other command in a single
session, sparing you the initialization costs for all but the first of them.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=188712815

java/google/registry/tools/RegistryTool.java

@@ -132,6 +132,8 @@ public final class RegistryTool {
 
   public static void main(String[] args) throws Exception {
     RegistryToolEnvironment.parseFromArgs(args).setup();
-    new RegistryCli().run("nomulus", args, COMMAND_MAP);
+    try (RegistryCli cli = new RegistryCli("nomulus", COMMAND_MAP)) {
+      cli.run(args);
+    }
   }
 }