Add Jsonable and AbstractJsonableObject for easier RDAP object building

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=246345611

java/google/registry/rdap/Jsonable.java

@@ -0,0 +1,23 @@
+// Copyright 2019 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.rdap;
+
+import com.google.gson.JsonElement;
+
+/** Designates something that can be turned to a JSON format. */
+interface Jsonable {
+
+  JsonElement toJson();
+}