Implement VKeyConverter (#538)

* Implement VKeyConverter

Implement a SQL converter that can be used for VKey objects.

Caveats:

- This only works with string columns (there's an excellent chance that all of
  our VKeys will use SQL string columns).
- Using this dpesn't establish a foreign key constraint between the referenced
  type (the "T" in VKey<T>) and the entity itself: this needs to be
  defined manually in the schema.

core/src/main/java/google/registry/persistence/converter/VKeyConverter.java

@@ -0,0 +1,41 @@
+// Copyright 2020 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.persistence.converter;
+
+import google.registry.persistence.VKey;
+import javax.annotation.Nullable;
+import javax.persistence.AttributeConverter;
+
+/**
+ * Converts VKey to a string column.
+ */
+public abstract class VKeyConverter<T> implements AttributeConverter<VKey<T>, String> {
+  @Override
+  @Nullable
+  public String convertToDatabaseColumn(@Nullable VKey<T> attribute) {
+    return attribute == null ? null : (String) attribute.getSqlKey();
+  }
+
+  @Override
+  @Nullable
+  public VKey<T> convertToEntityAttribute(@Nullable String dbData) {
+    return dbData == null ? null : VKey.createSql(getAttributeClass(), dbData);
+  }
+
+  /**
+   * Returns the class of the attribute.
+   */
+  protected abstract Class<T> getAttributeClass();
+}