Add XML support for new Flags EPP extension

For the .app discounting logic, we need a new extension which will let
registrars set, clear and query custom flags on a domain. Hopefully
this will be reusable for other custom TLDs later. This CL adds the
XSD, the associated classes for marshalling and unmarshalling, and some
marshalling tests, and links the classes into the system-wide extension
lists.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=128178999

java/google/registry/model/domain/flags/FlagsCreateCommandExtension.java

@@ -0,0 +1,33 @@
+// Copyright 2016 The Domain Registry 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.model.domain.flags;
+
+import google.registry.model.eppinput.EppInput.CommandExtension;
+import java.util.List;
+import javax.xml.bind.annotation.XmlElement;
+import javax.xml.bind.annotation.XmlRootElement;
+
+/**
+ * A flags extension that may be present on domain create commands. The extension specifies one or
+ * more flag strings. The extension does not dictate the use of any specific flags, but leaves it up
+ * to the particular TLD-specific logic. Some TLDs have special rules regarding discounts and
+ * eligibility. Such TLDs can define whatever flags they need to use for interacting with the
+ * registrar, and pass them using the flags extension.
+ */
+@XmlRootElement(name = "create")
+public class FlagsCreateCommandExtension implements CommandExtension {
+  @XmlElement(name = "flag")
+  List<String> flags;
+}