Add a frontend endpoint for retrieving a domain in JSON form (#1916)

We might (likely will) modify some of the fiddly bits around this (maybe
the GSON serialization, where we do the actual authorization, etc) but
this should be a decent basic shell structure for endpoints that the new
registrar console can call to retrieve JSON results.

util/src/main/java/google/registry/util/DateTimeTypeAdapter.java

@@ -0,0 +1,41 @@
+// Copyright 2023 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.util;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.util.Objects;
+import org.joda.time.DateTime;
+import org.joda.time.format.ISODateTimeFormat;
+
+/** GSON type adapter for Joda {@link DateTime} objects. */
+public class DateTimeTypeAdapter extends TypeAdapter<DateTime> {
+
+  @Override
+  public void write(JsonWriter out, DateTime value) throws IOException {
+    out.value(Objects.toString(value));
+  }
+
+  @Override
+  public DateTime read(JsonReader in) throws IOException {
+    String stringValue = in.nextString();
+    if (stringValue.equals("null")) {
+      return null;
+    }
+    return ISODateTimeFormat.dateTime().withZoneUTC().parseDateTime(stringValue);
+  }
+}

util/src/main/java/google/registry/util/UtilsModule.java

@@ -16,6 +16,8 @@ package google.registry.util;
 
 import com.google.appengine.api.modules.ModulesService;
 import com.google.appengine.api.modules.ModulesServiceFactory;
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
 import dagger.Binds;
 import dagger.Module;
 import dagger.Provides;
@@ -25,6 +27,7 @@ import java.security.SecureRandom;
 import java.util.Random;
 import javax.inject.Named;
 import javax.inject.Singleton;
+import org.joda.time.DateTime;
 
 /** Dagger module to provide instances of various utils classes. */
 @Module
@@ -83,4 +86,13 @@ public abstract class UtilsModule {
   public static StringGenerator provideDigitsOnlyStringGenerator(SecureRandom secureRandom) {
     return new RandomStringGenerator(StringGenerator.Alphabets.DIGITS_ONLY, secureRandom);
   }
+
+  @Singleton
+  @Provides
+  public static Gson provideGson() {
+    return new GsonBuilder()
+        .registerTypeAdapter(DateTime.class, new DateTimeTypeAdapter())
+        .excludeFieldsWithoutExposeAnnotation()
+        .create();
+  }
 }