Support multiple versions of the EPP Fee Extension

We want to support multiple versions of the fee extension, to allow new features while maintaining backward compatibility. This CL extends the framework and adds one new version, 0.11 (spec version 7), to the existing version 0.6 (spec version 3). A follow-on CL will add version 0.12 (spec version 8).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127849044

java/google/registry/model/domain/fee/FeeCheckCommandExtensionItem.java

@@ -0,0 +1,32 @@
+// 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.fee;
+
+/**
+ * Interface for individual fee extension items in Check commands. These are derived from the more
+ * general query items (which cover Info commands as well), but may also contain a domain name,
+ * depending on the version of the fee extension.
+ */
+public interface FeeCheckCommandExtensionItem extends FeeQueryCommandExtensionItem {
+
+  /** True if this version of the fee extension supports domain names in Check items. */
+  public boolean isDomainNameSupported();
+
+  /** The domain name being checked; throws an exception if domain names are not supported. */
+  public String getDomainName() throws UnsupportedOperationException;
+  
+  /** Create a builder for a matching fee check response item. */
+  public FeeCheckResponseExtensionItem.Builder createResponseBuilder();
+}