Add support for a domain transfer request superuser EPP extension

Allow superusers to change the transfer period to zero years and allow
superusers to change the automatic transfer length.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=167598314

java/google/registry/model/domain/superuser/DomainTransferRequestSuperuserExtension.java

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+// 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.model.domain.superuser;
+
+import google.registry.model.domain.Period;
+import javax.xml.bind.annotation.XmlElement;
+import javax.xml.bind.annotation.XmlRootElement;
+
+/** A superuser extension that may be present on domain transfer request commands. */
+@XmlRootElement(name = "domainTransferRequest")
+public class DomainTransferRequestSuperuserExtension extends SuperuserExtension {
+  // We need to specify the period here because the transfer object's period cannot be set to zero.
+  @XmlElement(name = "renewalPeriod")
+  Period renewalPeriod;
+
+  // The number of days before the transfer will be automatically approved by the server. A value of
+  // zero means the transfer will happen immediately.
+  @XmlElement(name = "automaticTransferLength")
+  int automaticTransferLength;
+
+  public Period getRenewalPeriod() {
+    return renewalPeriod;
+  }
+
+  public int getAutomaticTransferLength() {
+    return automaticTransferLength;
+  }
+}

java/google/registry/model/domain/superuser/SuperuserExtension.java

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+// 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.model.domain.superuser;
+
+import google.registry.model.ImmutableObject;
+import google.registry.model.eppinput.EppInput.CommandExtension;
+import javax.xml.bind.annotation.XmlTransient;
+
+/** Base class for superuser EPP extensions. */
+@XmlTransient
+public abstract class SuperuserExtension extends ImmutableObject implements CommandExtension {}

java/google/registry/model/domain/superuser/package-info.java

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+// 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.
+
+@XmlSchema(
+    namespace = "urn:google:params:xml:ns:superuser-1.0",
+    xmlns = @XmlNs(prefix = "superuser", namespaceURI = "urn:google:params:xml:ns:superuser-1.0"),
+    elementFormDefault = XmlNsForm.QUALIFIED)
+@XmlAccessorType(XmlAccessType.FIELD)
+package google.registry.model.domain.superuser;
+
+import javax.xml.bind.annotation.XmlAccessType;
+import javax.xml.bind.annotation.XmlAccessorType;
+import javax.xml.bind.annotation.XmlNs;
+import javax.xml.bind.annotation.XmlNsForm;
+import javax.xml.bind.annotation.XmlSchema;