Fix #759 Add abstract getText() method to the TextObject class (#801)

Author: seratch

slack-api-model/src/main/java/com/slack/api/model/block/ContextBlockElement.java

@@ -13,4 +13,7 @@
  * Block's elements</a>
  */
 public interface ContextBlockElement {
+
+    String getType();
+
 }

slack-api-model/src/main/java/com/slack/api/model/block/LayoutBlock.java

@@ -16,4 +16,10 @@ public interface LayoutBlock {
      * and image.
      */
     String getType();
+
+    /**
+     * Returns the block_id string; the value can be null if the object is manually crafted.
+     */
+    String getBlockId();
+
 }

slack-api-model/src/main/java/com/slack/api/model/block/composition/TextObject.java

@@ -6,4 +6,7 @@
  * https://api.slack.com/reference/messaging/composition-objects#text
  */
 public abstract class TextObject implements ContextBlockElement {
+
+    public abstract String getText();
+
 }