Support multi image metadata (#694)

Author: tawAsh1

line-bot-model/src/main/java/com/linecorp/bot/model/event/message/ImageMessageContent.java

@@ -38,4 +38,5 @@ public static class ImageMessageContentBuilder {
 
     String id;
     ContentProvider contentProvider;
+    ImageSet imageSet;
 }

line-bot-model/src/main/java/com/linecorp/bot/model/event/message/ImageSet.java

@@ -0,0 +1,50 @@
+/*
+ * Copyright 2021 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you 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 com.linecorp.bot.model.event.message;
+
+import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
+import com.fasterxml.jackson.databind.annotation.JsonPOJOBuilder;
+
+import lombok.Builder;
+import lombok.Value;
+
+@Value
+@Builder(toBuilder = true)
+@JsonDeserialize(builder = ImageSet.ImageSetBuilder.class)
+public class ImageSet {
+    @JsonPOJOBuilder(withPrefix = "")
+    public static class ImageSetBuilder {
+        // Providing builder instead of public constructor. Class body is filled by lombok.
+    }
+
+    /**
+     * Image set ID. Only included when multiple images are sent simultaneously.
+     */
+    String id;
+
+    /**
+     * An index starting from 1, indicating the image number in a set of images sent simultaneously.
+     * Only included when multiple images are sent simultaneously.
+     */
+    Integer index;
+
+    /**
+     * The total number of images sent simultaneously.
+     * Only included when multiple images are sent simultaneously.
+     */
+    Integer total;
+}