Prepare index type annotation

Author: greenrobot

objectbox-java-api/src/main/java/io/objectbox/annotation/Index.java

@@ -27,6 +27,11 @@
 @Retention(RetentionPolicy.CLASS)
 @Target(ElementType.FIELD)
 public @interface Index {
+    IndexType type() default IndexType.DEFAULT;
+
+    /** Only allowed for {@link IndexType#VALUE} and types String and byte[]. */
+    int maxLength() default 0;
+
 //    /**
 //     * Whether the unique constraint should be created with base on this index
 //     */

objectbox-java-api/src/main/java/io/objectbox/annotation/IndexType.java

@@ -0,0 +1,21 @@
+/*
+ * Copyright 2018 ObjectBox Ltd. 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 io.objectbox.annotation;
+
+public enum IndexType {
+    DEFAULT, VALUE, HASH, HASH64
+}